duel 3" sewer connections using a Sewer Solution?

I have the same set up. 1" PVC fittings fit the Sewer Solution so I made a "Y" connector from PVC with a standard water hose fitting connected to the dump end fitting, put the sewer solution on the black/grey dump, installed a 3" cap with a hose fitting on the other gray tank dump and run a standard water hose from it to the "Y". When I start to dump the black I disconnect the hose at the "Y" and cap the "Y" to eliminate the chance of black water entering the gray hose/tank.
Works like a champ.

The elbow connector on the discharge end of the Sewer Solution hose fits 1" PVC pipe fittings, so it certainly is possible to make up a wye type connector for two Sewer Solutions. Hershey is correct that a Sewer Solution is not needed for gray water, which has no large solids and will drain easily by gravity. A 3" cap with a garden hose connection would work fine for gray water.

I have not had to deal with your setup but I do use the Sewer Solution. I would think you would need two sewer solutions on the black tank drains and just a simple gravity hose to the gray water drain.
I'm guessing that it would be easier to just drain each hose one at a time rather than trying to retrofit a T or T's. You probably only need to drain the blacks once in a couple weeks, takes me three weeks but you would need to have a continuous flow from the gray tank.