Using the following source code:
It appears the "right" SID is mapped at $D400 which is where the C64 SID chip is mapped (meaning GT files should play without any changes, would need to test this though), and the "left" SID is mapped at $D440.
This means to play a SID track on the "left" SID, you need to add $40 to each address in the INIT and PLAY subroutines. Whether you do this to your SID file, or if you patch this as you load the file into RAM is entirely your choice.
Anyway, there's not much more I can say here; maybe deft or gardners or somebody else who's actually got their dev board up and running can confirm what I've said, or even correct me if I'm wrong.