You might want to share your goals/objectives as to wanting to connect the two. And maybe some details as to locations, sizes, etc. The more information you can provide, the better the advice you will get.
My uncle did this way back before he graduated form university. He had two pipes connecting two 100G tall tanks. I think they may have been made of acrylic but I remember they were blacked out for cleaning purposes... Cleaning a tight cylinder can be a pain.
The goal I would like to do is hook up another tank to my 180 gallon DT and use the same sump...So the two tanks share the same sump...Any pictures or drawings,, welcome..