> I am interested in adding additional AD/DA channels via the new ADI2-FS when it is released.

To the ADI-2 Pro ? So you talk about 1 device / 2 channels more ?

Then the signal flow would be like this:

Case A: Micro1 - Preamp ------------------------------------------------------+ Analog 1 IN - ADI-2 Pro
Case A: Micro2 - Preamp ------------------------------------------------------+ Analog 2 IN - ADI-2 Pro
Case B: Micro3 - Preamp -----+ Analog1 IN - ADI-2 FS - ADAT OUT ----+ ADAT      IN - ADI-2 Pro
Case B: Micro4 - Preamp -----+ Analog2 IN - ADI-2 FS - ADAT OUT ----+ ADAT      IN - ADI-2 Pro

Case A: you would have @44.1 kHz AD converter latency of 0.11 ms according to the manual.
Case B: you have the AD conversion only at the ADI-2 FS via ADAT the signal is already digital.

Assumption 1: to be confirmed by RME: potentially same A/D converter, so I would guess also 0.11ms.
Assumption 2: a little bit more on top because of the additional transfer via ADAT, but I would guess this is not much.

First the latency depends on the used filters. The ADI-2 FS uses SD Sharp for AD (and DA), and the same chip(s), so the latency will be nearly identical if you set the same filters on the ADI-2 Pro.

Second the additional path to digital out and back in will add a few samples for digital buffering. I just measured it via USB, feeding both units the same analog signal:

Single Speed: 44.1 kHz - 52 us = 2 samples
Double Speed: 96 kHz - 23 us = 2 samples
Quad Speed: 192 kHz - 38 us = 7 samples

I don't think such a miniscule delay needs any kind of interaction when talking about microphone recordings. Just make sure the main stereo mics (or the ones that relate to a main signal) are on the same unit.

For example setting the ADI's AD filter to Sharp instead will cause 16 samples difference in single speed.