I'm able to create a seamless split with many sound combinations. but sometimes (with say a Synth Lead sound on R and bass on L) I set the split point and the bass gets all the way there but the synth part is silent for 5-6 notes nearest to the bass part. then, usually on C, it kicks in. what setting am I missing? thanks

I assume R is Right and L is Left... although these are not the terms used in the MX

The MX uses Part 1 = Right hand, Part 2 = Left hand.
If there are Voices in these two Part Slots and you press the [SPLIT] button, Part 1 will be right hand, and Part 2 will be left hand.
Hold [SPLIT] and touch the Key you wish to be the lowest note of the right hand sound.

Thanks and yes that's correct re the parts. what I'm finding is that on some Performances there's a small "dead zone" -- Part 1 follows the split point I set but Part 2 starts sounding a few notes up from my split point.

hope that makes sense and thanks

Part 1 follows the split point I set but Part 2 starts sounding a few notes up from my split point.

But Part 2 is the left hand. (?)

Yes, it is. pls excuse my transposing Pts 1 and 2. the issue, which does not happen all the time, is a small zone between part 1 and part 2. when I go to Edit Part 1, it takes the split point. when I attempt to play the split, I sometimes have a zone of 3-5 halfsteps with no sound at all. same result if I Edit Part 2