Propane grill and Blackstone griddle with one 20# tank

Here's one solution available on Amazon:

Stansport 3 Outlet Propane Distribution Post

They make a handy dandy "Y" connector. two styles

one is ACME connectors all the way around if I'm not mistaken

The other is Acme to the Tank and "Quick Connect" (like throw away bottles)

Most major RV places and Propane Stores (not just a "Gas station" but a full Propane store) will have 'em.

The following link is NOT a recommendation as I just did a quick google search (Propany Y Splitter)

Propany Y Splitter on Amazon

I run a coleman stove and my blackstone off one tank. I use the regulators that the stove and blackstone came with. Here is a tee adapter and the hose I use to connect to the oem regulator. You will need two hoses. I don't know what kind of grill you have so you may have to adapt it to a regulated hose. You don't want two regulators on a single appliance.
tee
hose
This works for the 17" and 22" Blackstone that uses 1lb bottles. I don't know what kind of regulators the larger Blackstones have.

Just search for propane hoses. I bought an 18 footer that way. They have all different lengths.

You can buy one regulator with two pig tails.