So I can't figure out how to record one track at a time without losing previous recording..

Did you use 'Overdub' for 'RecordType' for the second recording?

See page 64 in the Reference doc.
https://usa.yamaha.com/files/download/other_assets/4/1583934/modx_plus_en_rm_a0.pdf

Record Type
Determines the Recording Type. This parameter is not available for the first recording.
Settings: Replace, Overdub, Punch
Replace: You can use this method when you want to overwrite an already recorded Track with new data in real time.
The original data will be erased.
Overdub: You can use this method when you want to add more data to a Track that already contains data.
Previously recorded data will be maintained*.
Punch: You can use this method when you want to overwrite data to a specified range of a Track that already
contains data. It allows you to overwrite the already recorded data from the starting point to the ending point
(measure/beat) that was specified before recording.
* When using Overdub with controller events that exist in the original recording, the existing controller events are
disabled and deleted,

See all of those ON lights at the bottom of the screen. They show you what recorded parts will
play while you record new stuff.

1. Record part 1 then stop the recording
2. press the record button to get ready for part 2
3. Set 'RecordType to 'Overdub' so that the previous recording is preserved. If you use
the default 'Replace' then the 2nd recording will replace the 1st one. That 'RecordType'
isn't available for the 1st recording.
4. Record part 2 then stop the recording
5. Rewind and then press the 'play' button. You should hear both recordings.

Test the above with a VERY SIMPLE recording of just a few notes for each part until you
get the right results.

Maybe you were confusing 'tracks' with 'recordings'. When you record the keys you hit and
the velocity you hit them with will determine which parts will record to each parts 'track'.

The recording will contain notes for ALL parts/tracks that were used based on the keys/velocity
you used.

In MIDI mode you are recording the performance - not parts or tracks. Notice on that screen that
the keyboard icons show you which parts are keyboard enabled but you can't change them from
that screen - and if you exit the screen to change them.

Part 1 = Piano
Part 2 = Bass

I would first turn keyboard control off of every Part 1-8. Then select Part 1 to record track 1. Then select Part 2 to record track 2. Using overdub can be done but if you have keyboard control on for all of your Parts then the overdub will mix into all of the Parts.

https://www.youtube.com/live/FFDil5y5SI0?si=6s6w6qqRY_LPIy1J

Applies to MODX(+) too. Notice the second instrument he records turns keyboard control off for every Part except for that one instrument's Part.

It's worth saying if you had a multi-Part piano then I would start by turning keyboard control on for those 2-4 Parts (however many the Piano uses) and turn off keyboard control for every Part when recording single Part sounds like a bass (or even certain pianos).

Using overdub can be done but if you have keyboard control on for all of your Parts then the overdub will mix into all of the Parts.

When I do that using 'Replace' it wipes out the first recording.

I'm not sure you were catching what I was throwing. You can use overdub but make sure to manage what "tracks" (Parts) you want to record. If you tell MODX to record all Parts by turning on Keyboard control for everything then you run the risk of recording to tracks you don't want data on. And the next recording overdub will put data on all of those same tracks.

Splits with note ranges that don't overlap is one way to manage but the suggestion I have to use keyboard control works for everything. Layered or split.

So ..

Overdub + Keyboard control on Parts 1-8: bad (practice)

Overdub + Keyboard control on for only the parts you want to record: good

You can use overdub but it bites you if not managing your overdubs with keyboard control .

You can use overdub but make sure to manage what "tracks" (Parts) you want to record. I

I noticed you said you 'can use overdub' and not 'when you use overdub'.

Isn't it necessary to use overdub to prevent OPs problem? I agree you need to also manage the keyboard control for parts 1-8.

Do you agree that when selecting/recording parts 9-16 you do NOT need to disable keyboard control for parts 1-8?.

Just as an aside I'm not aware of any way to control things by 'track' like OP was suggesting. Is there a way to do that using MIDI record?

I used it in a manner like: "you can go outside, but wear your jacket because it's raining".

Anyways - technically when recording single Part sounds you only need to turn keyboard control off that one Part and the rest don't matter since they (all Parts with keyboard control on) will be ignored when a Part with keyboard control OFF is selected. So if you're recording 9-16 then it doesn't matter what Parts 1-8 are set to.

However, if you always record single Part instruments then I would always turn keyboard control off of every Part or, more efficiently, use the Multi/GM initialized preset Performance. It disables keyboard control from every Part except for Part 1. This is only a little risky because if you select no Parts (like what happens when you press [PERFORMANCE] (HOME) ) then now you'll be recording Part 1 which may not be intended. Turning Keyboard Control off of every Part is, to me, good general advise that will work for every situation of recording single Part sounds.

Any time you want to record a multi Part sound you should always record it in Parts 1-8 (with keyboard control turned on for every Part of this sound) then move these Parts somewhere in 9-16 when you're done.

The OP seemed to suggest using single Part sounds so the advice was geared towards that and then also added mention of what to do in the case of multi Part.

Tracks ...

"Tracks" is a MIDI file term. If you look at the binaries generated by MODX (not only MIDI but library, user, and backup files too) you'll see track (or trk) in there for certain MIDI-file-ish data. MIDI files use the MIDI protocol but also adds some different conventions and nomenclature.

A track in a MIDI file maps to a MODX part.