Technically possible; Unisub at one time had instructions detailing how to do both sides of aluminum ornaments at once. The downside of this approach is the difficulty in getting exact placement between both sides. It probably could be done but adds more margin for error.

I mirror one side and I don't mirror the back side.

150 seconds seems far too long, but I've never used A-Sub personally. We use Beaver Paper's Textprint-R which will have different ink retention and release characteristics. I've only observed going above 1:15 on metal with large format blanks ie 40" x 60" or 48" x 48". If you have a smaller cheaper blank like a bookmark (thinner and meant to bend), I'd try that first.

I tried 400 degree for 30 seconds and the center is still coming out a bit lighter than it should. Any suggestions?

@@agodinez1954 Try more pressure on it.

We're using Textprint-DT. It absorbs a high amount of ink but also releases it accurately without out-gassing through the back of the paper.

You'd probably want to keep it consistent with other sublimation metals in the press and your ink. Possible limiting factor may be pressure: ~30-40 lbs/square inch is about what we're using.