There's no doubt that the rear motor will be running at less than maximum efficiency due to a number of factors. First will be due to operating in the wake of the forward prop. And second will be picking up turbulent air off the fuselage and wing.

But there's not going to be any easy way of calculating this ahead of time. There's simply no good way of estimating the effect on the rear motor.

All you can do is finish the model and try some tests with one motor and two motors running and see what the current to each is like and what the overall static pull is like. But static thrust measurements on such a model don't really mean a whole lot if you're using a prop with a pitch that is much over 4".

So you're stuck with running some inflight testing that uses different pitch value props on front and rear until you find a combination that works. No promises but it's likely that the rear prop will end up being slightly higher in pitch when then happy moment occurs.