TA-15 safe mismatch

I have two 16 ohm cabinets which I connect individually to the 8 ohm output with no problem. I'd like to use both at the same time. I tried it for a few minutes with one connected to the 8 ohm out (like I usually do-this is listed as a safe mismatch in the manual) and the other connected to the 4 ohm out (not listed in the manual). I shut it down quickly, not wanting to strain the output transformer, but it sounded good.
Anyone know if this is safe.
I would get a Y splitter and connect both to the 8 ohm out, thinking the amp would see 8 ohms that way, but I read it would need to be a Y splitter for speaker cable and this seems hard to find.
I would hope not to have to rewire the cabinets. There is a lot of conflicting info on this topic.

Thanks again

Definitly DO NOT use both outs like you did. Short term like you did shouldn't be a problem(Mesas are pretty tough), but long term would burn up your output transformer. This is what you need http://www.humbuckermusic.com/cuspcafordrz.html It is wired in parallel, with the 2 16ohm cabs it will give you an 8 ohm load.

Thank you very much.
I resisted the temptation to try it again until I learned more.
i'll order the Y cable today.