you can do whatever you want. the suggestion was for your convenience, it’s not a ‘can or can’t’ sort of thing.
I don’t see any issue with using the same needle in 2 vials, myself. the amount of cross contamination should be extremely minimal, you’re not pushing the first steroid into the second vial. you should just draw it. I’ve plunged a pin into a vial, removed, then re-plunged into the same vial without changing needles. I’m sure most people have. There’s no substantive difference between this and switching to a different vial. So in your case, if you draw the test first, then the tren H, you’ll contaminate the Tren H with a trace amount of the test… but who cares, right? it’s not going to matter if the next time you draw from the tren H bottle, you end up with trace amounts of test.
I can totally understand why people exercise a lot of caution with this stuff, but you really need to consider the net effects of your actions when deciding if something ACTUALLY matters or not. In this case, ‘contamination’ does not imply the transference of bacteria or anything unwanted from vial to vial. It’s just a little bit of a steroid into another steroid. No big deal. The vials will both be empty in a few weeks anyway, right?