I'm probably doing something wrong

I am recording in Seq mode track by track. I record voices (Copy from program) on tracks 1 & 2 and drums on track 10 I do this for 30 measures. I want to record a combi from measure 31 onwards. The combi uses tracks 1 & 2 and changes the voices recorded on those tracks for those used by the combi.
I tried to get over this by copying tracks 1 & 2 to 15 & 16 (they are not used by the combi) and erasing the original tracks 1 & 2. I thought this would allow the combi to record on it's allotted tracks (Copy from Combi) but no - it cancels tracks 15 and 16, they revert to no voice.
The only way round this in my little mind would be to record the combi first and then add the extra tracks (which I don't really want to do) unless you guys have any other solution.
Any help would be appreciated
Peter