Syncing 2 forest in Single Ad connect

This is a supported Topology. see the following article. https://docs.microsoft.com/en-us/azure/active-directory/hybrid/plan-connect-topologies#multiple-forests-single-azure-ad-tenant

Essentially in AD Connect you need to go in to the connected directories and put the other forest there. Then it can use Password Hash sync or whatever other option you want.

https://docs.microsoft.com/en-us/azure/active-directory/hybrid/how-to-connect-install-custom#connect-your-directories