ok, so, if you have a twincharge set up on a chevy small block 400 cu. in motor, turbo feeding roots blower...  roots by itself at 4000 rpm produces 6lbs boost...turbo by itself at 4000 rpm produces 15 lbs boost..what happens when we put them in series...turbo feeding roots at 4000 rpm ..what approximate boost will be get?...i'll know soon enough i'm building an engine..but would like some theoretical input.

You will get more total boost than just adding the two pressures.  The reason being that the pressure ratios multiply, not add.

So if your turbo has a pressure ratio of 2.0 (14.7 psi boost), and your supercharger has a pressure ratio of 1.4 (5.9 psi boost) you get a total combined pressure ratio of 2.8 which is 26.5 psi, not 14.7 + 5.9 = 20.6 psi.

So expect to see 14.7 out of the turbo, and 26.5psi out of the supercharger. The supercharger will work A LOT HARDER with much denser air being forced into into it.

The best way to handle this is to just run your wastegate sensing line off the total combined boost, and select a wastegate spring for whatever total boost you finally decide to run. Boost will quickly rise to that pressure and stay there over a very wide Rpm range.

If your exhaust housing originally produced full boost at 4,000 Rpm with just the turbo, expect full boost at more like 2,000 Rpm with twincharging. I am not kidding.  You will definitely require a larger a/r exhaust housing, but try it first and see.

The whole thing will be far more responsive and have a much lower turbo boost threshold than you are probably expecting. But the top end power will still be there.

With boost pressure substantially higher than total exhaust back pressure, over a very wide Rpm range, it is going make some real horsepower, as well as having massive low end grunt.

But this has a couple of things going for it.  The supercharger pressure ratio is reasonably low at only 1.4, and if some fairly efficient intercooling is employed after the turbo, the temperature at the supercharger intake need not be very far above ambient.

The detonation problem that often plagues high boost turbo engines is usually more often due to intake charge heating by trapped exhaust residuals. The unavoidable high turbine back pressure, traps a lot of heat, especially where there is a low compression ratio.

With supercharging, or twincharging it is extremely easy to keep boost pressure well above total exhaust back pressure, and with sufficient valve(Reducing Valves) overlap, the heat can escape due to much more efficient positive scavenging. With EFI, the injection point can be delayed until the exhaust valve has closed if a good fuel specific is required.

So the engine is going to have a far higher detonation threshold than can be had with a turbo.

High induction temperatures are never good, but at least with supercharging or twincharging, detonation should be far easier to avoid with some suitable (but not excessive) valve overlap.