I once had the same concern with B12 and GH (was it ok to dilute growth powder into sterile water based B12?), so I asked Gensci China directly (must have been about 5 or 6 years ago, Jintropins), and a physician answered it was a no-no due to (if memory serves) the sodium chloride listed as an ingredient in the B12, and the difference in PH it was inducing... the GH would have been damaged, thus ineffective.
I assume it's the same in your case with Synthetine and HCG.