If I read manual ch 12.2 of the ADI-8 QA correctly, then you can activate the option "digital to digital" in the setup.
Then it should be possible to route the primary ADAT input port to e.g. one of the 8 MADI blocks.
But the 2nd ADAT input port can not be used for a 2nd device, this is for multiplexing of channels, if double speed is needed (88.2/96).
So from what I see you can only add either the FF400 or the ADI-2 to ADAT in and route it digitally to MADI.
In this case you would need a Firewire connection to preconfigure routing on the FF400, so that you can route some line inputs for A/D to ADAT so that this will be transfered to the ADI-8Qs and further via MADI to the MADIface Pro.
For the ADI-2 I see no real use case in regards to the amount of channels, because it offers to you only 2 analog channels.
Compared to that the FF800 offers in total up to 8 line inputs (including Mic, Inst, Line inputs).
After having configured routing on the FF400 (using a PC with Firewire) you can store the settings and from then on use the device in stand-alone mode, according to FF400 manual ch 20.
I hope I made no stupid mistake after making a quick ride through the two manuals... this is how I see the situation.
The alternate solution would be to get e.g. a 2nd ADI-8QS if you want the same device again.
Alternatively there are meanwhile other AD/DA converter avalable but they have their price ...
M-32Pro AD and DA and M1610-Pro.
An option could be, if price is the main driver for you, to look at AD/DA converter from the german brand ferrofish which are also being offered by Synthax AFAIR (Ferrofish Pulse 16 MX or even Ferrofish A32, but with the latter you would need additional patch bays).
If you need sample exact recordings it could possibly an advantage to stay with one or more ADI-8QS.
Having two of the ADI-8QS could also be an advantage in other regards, because those AD/DA converter allow reference level settings only for all 8 ports. So if you need a mix of consumer and studio level, then you need two different of those devices anyway or make some adjustments using digital gain.
Devices like the M-32 Pro offer reference level settings per port.
So you might need to look closer what you really require. Otherwise the ADI-8 QS is a fine unit. Nice setup.
X10SRi-F, E5-1650v4, Win10Pro20H2, Cub11Pro, UFX+ (v0.9735), XTC, 12Mic, ADI-2 Pro FS R BE