Yep 2 grow tents can be connected. the grow tents made by secret jardin are connectable. or you could just use their mammoth grow tent. heres a link http://www.secretjardin.com/
secret jadens are expensivs as hell. On eby you might be able to find something good. I got mine on ebay from californa nurseries and paid $229 for it, i looked ar worms way at a secret jarden and these 2 are identical but the Secret Jarden was $1299.
BigBudE
If your dead set on joining 2 together you can go to Sherwin williams paint store and in the drywall section they have instant doors using a long zippers for plastic walls. Its kind of hard to explain but but imagine a the wall on your grow tent and imagine 2 zipper strips that make a door. Hope that helps
BigBudE