Why not? You can do that in Chainer.No way to take mutiple MIDI channels and route them to ST2's 16 instrument slots on one Chainer channel.
Do this:
o Open Chainer
o Load an instance of Sampletank 2 in a Chainer slot
o Load an instrument in channel 1 of Sampletank 2
o Load a second instrument in channel 2 of Sampletank 2
o Go to the lefthand pulldown menu (the menu item marked by a down arrow) on the Chainer slot that holds the Sampletank 2 instance
o Select "Midi transmit channel => All"
After doing the above, midi that you send on channel 1 will go into Chainer and then into your first instrument in Sampletank 2. Midi on channel 2 will go to your second instrument in Sampletank 2. I just tried it with Sampletank 2 and it works great.
Is this want you need or am I misunderstanding your intent?