In Renoise, an instrument is tied to one midi channel, but when you load different sounds into SampleTank, they get assigned to different midi channels.
So does this mean I can only play 1 sound per ST instance and I have to load a new ST instance in a new instrument for every sound I want to use?

check out plugin aliasing feature; this is what solves your problem

I think what you’re asking is how to play two or more sounds in unison? (ie piano + organ)
on the left side of the window there’s the column that reads “ch.” and has midi channels 1 through 16 on it. You can click on one of these values and drag up or down to change it. Set each column to 1 if you want to be able to play different sounds at the same time.

Thanks It-Alien, it works now.