The MX is based on the Motif sound set. Basically, it is a scaled down version of the Motif - you can place Voices into Performances. A Voice is placed in a Performance PART (the PART is a set of parameters that are applied to the Voice as offsets to the Voice)... The amount of editing you can do via the front panel is some what restricted compared to its bigger siblings (MOXF and Motif XF). You can combine 16 Voices together into Performance with limited amount of offset editing to those sounds. The full engine is there and available for editing via MIDI. Functions in the deeper areas of Voice Editing (shown in pink in the manual margins) require either an EDITOR or use of System Exclusive message.

This can be done with System Exclusive messages found in the MX49/61/88 Data List Booklet or with the John Melas MX Tools (The Total Librarian includes a Voice Editor, Performance Editor and a Drum Kit Editor).

These allow you to get to all the parameters not available via the front panel.

Even if you do not have this Editor... you should be able to get to the parameters you need to customize Portamento Time. You can select a Synth Lead Voice like "Lucky" (Lucky Man synth lead sound) or "Rap 1", both have portamento programmed in... and more than likely have Portamento Time pre-assigned to one of the Assign Knobs. In order for you to program that parameter to a Voice you'd need to address it with MIDI messages. Are you using the Melas MX Total Librarian? If you want to unlock all the power of the MX (it is highly recommended).

But I believe both the Portamento Switch and Portamento Time are available for every PART in a Performance. These can be used to apply Portamento to just about any sound (except Drums)

It's also unclear as to conflicts between the Voice Play Mode Porta time and the other time amount set in the Voice Ctrl Set for the assignable controls.

We are sure you mean the Performance PART "Play Mode" Portamento Time and the one listed in the Voice Control Set... The one listed in the Performance PART "Play Mode" area applies an overall offset to whatever Voice gets assigned to that PART. The Voice Control Set listing of the Portamento Time is the specific detail of how and where it is assigned... whether it is FULL TIME or FINGERED behavior. So on Voices like the Synth Lead category, you would expect that Portamento might have been addressed in some detail (at the Voice level of editing). Try "Mini Soft" and "Feeling" unlike "Lucky" and "Rap 1" Portamento here is FINGERED - so only when you play legato will there be Portamento

A Performance at maximum can be 16 Parts, one on each MIDI channel. The PART Edit Portamento Time can be used to control/offset the PART you have selected to play. The Voice Edit parameters inside the CONTROL SET allow you to customize the behavior - select the controller assigned to apply Portamento Time for that individual VOICE.

A single Controller can control multiple destinations... that is entirely up to you. Multiple Controllers can be assigned to change a single parameter... again, that is entirely up to you. If you move both controllers simultaneously - that would not be good, but assuming you are not abusive, you can choose your weapon (one at a time).