Do IEEE transactions allow for any "co-first authors"?
Yes, it is allowed. Google scholar is your friend in such cases.

I employed this query, which looks for publications with "IEEE Transactions" in them, the text "contributed equally to", and one in (text | paper | work | article).

The results show several papers published in IEEE Transactions, which specify multiple equal co-authorship. The way this is specified seems to be non standardized.

For example, http://dx.doi.org/10.1109/TNB.2006.875054 specifies that "The ﬁrst two authors contributed equally to this work." in the space reserved for details such as when the manuscript was received and revised.

In http://dx.doi.org/10.1109/TED.2009.2034804, the equal contribution is specified in the ACKNOWLEDGEMENT section of the paper.

Both ways look good to me. The first case is likely achieved by communicating the authors' contribution in the cover letter / space for staff communication. The second case is achieved by simply writing the contribution in the text body.