Yes exactly that

If the 2nd Performance is a factory preset sound - then [STORE] a user version with an adjusted volume setting. If it's a user Performance - then edit the volume and [STORE] (overwrite) the volume. That's a general case.

Alternatively, you can use Live Set to recall the 2nd Performance and use the Live Set volume offset feature to match the 2nd Performance's volume.

All of this assumes that the 1st Performance - at the time of the SSS switch to the 2nd Performance - is always the same volume and not different volumes. If sometimes the 1st Performance switching to the same 2nd Performance will be very loud or sometimes very soft - then this won't work.

What you MAY be able to do is something different.

1) Use Envelope Follower to somehow try to adjust the PART(s) in the 2nd Performance to better match the sustained Performance. This is a bit messy since you'll have to use the Main output envelope - which will change as you play the 2nd Performance and there's not really a "latch" (meaning sample the envelope follower at one point, hold that value, and not change the offset).

2) Use compression effects to remove dynamic range in general so "everything" is better matched

Hopefully when the 1st Performance switches to the 2nd one - the volume is always generally at the same level - because that's easier to deal with.