Connecting two D8s to a Studio 192 can be done, but it isn't ideal because the D8s are not as versatile as the Studio 192 in regard to Clock Source/Sync. The D8s only have a BNC input so you cannot chain devices for BNC clock.
ADAT connections can pass Clock Source, but BNC is always preferred for Clocking.
So, connect the ADAT connections from both D8s to the ADAT inputs of the Studio 192 respectively.
Connect a BNC cable from the Studio 192 BNC output to a BNC input on a D8, say D8 # 2.
Now the Clock Source settings will get tricky.
Set D8 # 1 to Internal Clock.
Set the Studio 192 to External Clock from ADAT. Do this in Universal Control.
Set D8 # 2 to External Clock BNC.
With each device set to the proper Clock Source setting, all should work.
However, ideally, two DP88 are much better suited for the Studio 192. Or at least one of each. One D8 and one DP88. This would do just fine keeping Clock Source to BNC on all devices.