Using 2 different audio interfaces on Mac together

When Aggregating 2 audio interfaces on Mac.
How would you direct monitor (without latency) all the inputs ?
Can the mixer utilities of each interface be used ?

You do direct monitoring with your interfaces the way you normally would. Using an aggregate device makes no difference to your monitoring.

Normally I use the sound interfaces’ dsp mixer utility for monitoring with reverb , but with aggregating that’s unavailable

I have never successfully got 2 interfaces to work without issues.

I don’t understand what you mean by “unavailable”, but it’s quite common to combine audio interfaces into an aggregate device when working with a DAW on MacOS.