" There's two waste gates, so I guess the exhausts are in parallel?
i don't think that necessarily follows, though it does rather look as though the turbines -aren't- plumbed one into the other. if you want to control the boost of each stage separately [to equalize the workload], you need a wastegate on each turbine.
sudden thought: they're popoff valves, not wastegates.
I made a spreadsheet with some of the basic turbo calcs. With the TE06 and motor at 6000 rpm, one turbo would produce 20psi and r=2.4. A second TE06 at r=2.4 with input at 20psig makes 45psig output.
If I 'play' the calcs and assume a turbo can do atmospheric to 45psi in one step (for comparison purposes only) it would be r=4.1.
It seems very likely that two turbos in series probably is less than twice as efficient (since obviously a lot of energy is taken out of the exhaust gas by the first turbo) a total r=4.1, far less than the theoretical r=5.76, seems ball-park reasonable.
But 50 psi plus or minus has got to be the number, to get 700 hp from 182 ci.