Mac Aggregate Sound Cards

I use a 2408 mkIII Motu and also have an MR816x.

When overdubbing, I switch to the MR816x to have no latency monitoring. Otherwise, I need the I/O available on the MOTU piece.

I can make the two interfaces work at the same time as an amalgamation, but then the MR816x can not be switched to direct monitoring. Any ideas about how to get direct monitoring with the combined interfaces?

Thanks!