This should be doable either way but the board that the AR’s connect to will control the preamp gain. It’s going to be a combination of configuring inputs, outputs, and tie-lines. In the case you explained, you would connect AR’s to the Slink port on the SQ7 and install the extra Slink card in the IO port of the SQ7. SQ6 (FOH) SLink Port would Connect to SLink in IO port of SQ7 (Mon).
Monitor board (SQ7)
In the case you explained, the local input channels of the SQ7 (Mon) would be patched to the AR’s on the SLink port.
You would also patch the inputs to the I/O card via tie-lines
FOH board (SQ6)
You would patch inputs to the SLink channels (coming from the SQ7 tie lines)
You would patch outputs, mix out, LR to channels on the SLink (back to the SQ7) and configure a tie-line there to the SLink (AR)
So, in my opinion, this should work and the general flow is above. However, I have not operated exactly in this setup. Others may have more detailed comments/opinions.