assigning

Ive made some beats, created some loops and assigned them to songs for
sequence. I now decide I wanna put some different sounds over the top
of the bare beats from a different program. How can I get the mpc to play
the existing beats and allow me to jam the new sounds over the top ? It only seems to be one or the other. . . . . .

Load both programs into memory
Go to Program, select DRUM1, and use the jog wheel to assign one program here.
Now go to DRUM2 and assign the other program to this DRUM holder.
Now go to your sequence and assign the correct DRUM holder to the correct tracks and both programs will play.