I have a project where I'm doing a fair number of classic afro-cuban rhythms. Many of these beg for a rich mix of sounds that include bongos, guiro, maracas, shekere, etc. Obviously, the maracas and shekere are better when I play acoustic in and I'm willing to do that, but I don't have a guiro, a cuica, or bongos. I started doing this using the Roland TTS-1 sound set (I still have a SoundCanvas hardware set!) as it was familiar, but after using the Addictive Drums Session Percussion for other projects, I at least want to have the congas, timbales, block, etc from those sets. I started doing one and wanted to add agogo bells and bongos for appropriateness, and found that I still had open a track and build a parallel Step Sequencer for the TTS-1 instruments. 
 
Is there any way to have the channels for both synths in a single step sequencer instance? If so, I could just build it once and save as a pattern and then be good to go....

Drum Maps might just be your answer to this.

If not, what I do is build with TTS-1 then split the parts out to a few tracks buy clone/delete unwanted. It's pretty easy and fast if you highlight all the notes from the keyboard on the left of piano roll view... highlight  delete. Then start playing with different soft synth sounds. 
 
As example:  a standard drum kit all in one track. I do my timing / editing and stuff with this, When happy I clone it 4 times and then open the first one and delete everything but the kick, Open the second one and delete everything but the snare and so on.. 
 
 

There are several ways to do this.
One is in the step sequencer itself, the other by creating a drum map then loading that into the step sequencer.

Sending one MIDI track to two synths is no problem. Just select that track as the input to the various synth vsti/audio tracks.

If you set the synths to different MIDI channels you can then select which channel each step sequencer row outputs to. It is even possible go have several step sequencer tracks each playing the same note but outputting it on different MIDI channels if desired. The step sequencer help shows you how to set up the rows so that send the right note on the desired MIDI channel.

The drum map route has the disadvantage that you have to learn about creating drum maps and the learning curve can seem a bit intimidating at first. Once you get the hang of it though maps can be create very quickly and are permanently available to re-use as required. They can also be used in piano roll view if you find you need to do something more complex than the step sequencer allows. Again, they allow output to multiple MIDI channels.

My personal approach is to use drum maps where possible and once learned they offer a very powerful way to do all kinds of things with MIDI, going well beyond just putting drum names in slots rather than having to remember which note triggers which drum. On the other hand, if you've never made one and only need a quick answer for a single song then editing the step sequencer rows would be simpler.