We have two grey tanks. No issues with us. I used to use a Valterra wye fitting but don't bother anymore. I always just lay the hose directly on the ground and dump the grey/black first and walk the hose contents into the sewer drain. Then move the hose over to the single grey tank and dump.
As mentioned, a simple of equalizing the tanks is to connect the 2 tanks together with with a garden hose. Use a fitting like below. Different versions/makes available. The one in the pic is nice as it makes removing and re-installing the end caps easy on the hands.
