Might you plug the MX12 and the Squid separately into a MIDI Solutions MERGER box then MERGER Out to DT? Mind you, I’m not at all familiar with the two units.
Also, there are the iConnectivity X-series MIDI interfaces which allow one to selectively route and filter signals In from source devices and Out to destination devices.
The mioXM could be a solution to your problem too, just more expensive.
If your problem has something to do with conflicting clock signals from more than one source, then you might have to use a combination of MIDI Solutions and iConnectivity devices as I have had to do.
Good luck!