Maybe aggregation is the wrong term, its an ASIO driver accessing multiple recording interfaces, that are detected / supported by the driver.
From RME driver perspective even three cards would be supported, the question is whether your computer is capable to support that (mainboard design, DPC latencies). When using the RayDAT you need also to check, that your PC case has enough slots. Per RayDAT you need 1 PCIe x1 slot for the card plus one for the daughter card.
By using WC you do not need so "sacrifice" 1 ADAT ports for synchronization purposes between cards.
The WC module you do not need necessarily, only if other external devices need to be connected through WC.
There is an internal 3-pin connector for WC out and WC IN that you can use for that purpose, see manual ch 6.2:
This would save you a 3rd slot in the cases.
Some add-on information: the RayDAT is also a very nice product, as it offers to you also WC and other I/O ports that you need in a studio setup / project studio (AES, 2x MIDI I/O, ADAT4 witchable from ADAT to TOSLINK protocol). Its a full digital card, no analog I/O ports present.
Unique for the Digiface USB is, that all 4 ADAT ports can be switched from ADAT to SPDIF protocol, the only RME interface where this is possible on all ADAT ports.
X10SRi-F, E5-1650v4, Win10Pro20H2, Cub11Pro, UFX+ (v0.9735), XTC, 12Mic, ADI-2 Pro FS R BE