Does your drummachine/sampler have less than 16 instruments?
Use the free instrument slots of a Zaquencer Drumtrack with another drummachine!
Either have the first drummachine put all notes through to the next one (and each drummachine is set so that they react to different notes), or hook them up to Output A and B of the Zaquencer, and set your drumtrack to output on both A and B at the same time.
You have to make sure that your two drummachines/sampler are able to receive on the same channel, but on different notes (ignoring all the other notes), as both machines will be seeing the same range of notes from the Zaquencer.