I'm having a hard time visualizing what happens if you have 2 wastegates at different settings.
Say one is set at 7 and another you thought was 7 is actually 6.

How does the 7 side interact with the 6 side once they hit the manifold... Is it possible for the 7 side to push against the 6 side?

 

one spends faster than the other? they balance out to meet in between pressure? one waistgate opens and closes(flutters)? hell i dont know but it shouldn't be that way

 

simple, the lower pressure setting WG will do all the "work" until its overwhelmed and the pressure builds up to the point that the higher pressure setting WG gets in on the act.

One turbo will be worked a little harder than the other, since its applying more flow.

The simple solution to this is to use weak springs in both WG and use a single spring and ball boost controller feeding both of them. Then, no matter what inconsistancies there are between the two wastegates, they will be opened at the same rates. But honestly, unless the wastegates are way way off, it won't make much difference.