How to use tabs in Textmate 2 for files opened via Transmit
I had the same question a while back, stumbled across your question, but saw there wasn't an answer. Today, I was frustrated, so set about to figure this out.

http://write.byronrode.com/textmate-2-tabs-and-transmit/ has the answer, which is a bit too detailed for an answer here.

Let me know if it helps.