There is no proof that Bio-Spira fully cycles your tank. At best, it processes ammonia, but doesn't actually establish a bacteria bed capable of dealing with a heavy bio-load. There are no products out there that are documented as effective. Until they gaurantee their product with a full replacement policy, I consider it bunk. Heck--the Bio-Spira people won't even provide samples to stores so they can test it out...Confidence inspiring, don't you think?
If you can't cycle the tank (even fishless with a heavy seeding would take 10-14 days), then you're stuck doing daily testing and water changes to control the toxins. Understand that your other tank will be cycling as well unless you are removing the fish currently in there completely. Anytime there is a significant change in the amount of waste produced, the bacteria populations change. When the bio-load increase, the bacteria lag a bit behind, and ammonia/nitrites build up.