I don't think there is any synergism, as in Deca providing an anabolic activity that more testosterone would not. And if wanting to use only so much testosterone and so any addition needs to be something else, certainly there's nothing done anabolically by Deca that various other injectables won't do.
In your specific example, my estimate would be, about the same. If the combination with Deca had a little more anabolic effect, which is possible, this really wouldn't be from synergism but rather from there being not exact milligram for milligram equivalence. So for example let's say, hypothetically, that if we could measure really exactly we'd find that actually it takes 1600 or 1700 mg/week of testosterone to be exactly equal anabolically to 1000 mg/week testosterone plus 500 mg/week Deca. That wouldn't prove synergism, it would just suggest that where already using a gram per week of testosterone, it takes 600 or 700 mg of testosterone to give equal further effect as 500 further mg of Deca.