You could make a bracket shaped pipe that goes up out of the top of one tank, across and down into the other tank.
Think a shape like this ] only horizontal instead of vertical.
If you do canister filters, put the intake in one tank and the outflow in the other. If you do it right, you'll create a flow between the two that will easily join the two tanks. I've seen videos of this being done on youtube if you want to research it.