I personally would not do so. My understanding is slightly hypo water can be a stress reducer, but treatment levels of hypo already have a lot of stress and risk. Similarly treatment levels of Copper are stressful. Adding them together seems very high risk/stress, especially for a fish that might be sick already. Also adding copper without the ability to test it is ... more risky than being able to do so.
That's all not just because of the stress induction in the fish, but also each of those is going to cause issues with any bio filtration you have, which also makes it hard to manage.
Given you already did it, I'd personally decide how much you care that the tang makes it through. If you care a lot, I'd personally consider doing some water changes with exact matched salinity or a tank transfer to a copper-less tank, doing a drip acclimation during the move. If you care less, I'd buy a copper tester, keep it stable until your hypo is done, raise salinity, and then finalize copper.
I'd also go read humblefish forums for more advice and ask for guidance there as well.