Here is a look at the control systems of the sequential turbo system and the problems that may occur. Hopefully it will help out with a common set of problems.
Actuators and VSVs:
There are four VSV/actuator assemblies that control the sequential turbo operation:
Intake Air Control Valve (IACV)
Exhaust Gas Control Valve (EGCV)
Exhaust Gas Bypass Valve (EGBV)
VSVs switch manifold pressure (boost) to and away from actuators. The actuators open/shut valve butterfly flaps.
When this switches, boost generated by #2 turbo is allowed to join the intake stream
When it isn't switched, there is a small flap in the IACV assembly that allows any boost generated by #2 turbo as it prespools to join the intake stream, preventing turbo stall.
This allows exhaust gas to flow through #2 turbo, causing it to spin up. When it's shut, there is no gas flow through the impeller of #2 turbo and therefore it doesn't spin.
This allows some exhaust gas to bypass the EGCV and join the output of the first turbo. It serves two purposes - one, it prespools the second turbo by allowing some exhaust gas flow through it. Two, it controls the amount of exhaust gas going through the first turbo in much the same way as a wastegate - therefore controlling the maximum boost pressure generated by #1 turbo.
This bypasses exhaust gas from the first turbo, lowering the overall amount of exhaust gas going through *both* turbos and controlling the maximum boost pressure generated by both turbos in parallel.
What it should be like:
You get 0.7bar of boost on turbo 1, a slight dip in power between 3500 and 4000rpm, and then 0.8bar of boost with both turbos online. This should be a smooth and linear power delivery.
When modified with a boost controller/decat, you'll see an increase in boost after 4000rpm when the second turbo comes online. You may also get 0.8bar of boost on the first turbo with a decat. You'll feel the same power dip at 3500rpm but then a noticeable surge in power at 4000rpm when the second turbo comes online and hits over 1 bar of boost. Full boost is acheived before 4500rpm.
When things go wrong:
Problem: I don't get a 2nd turbo rush, it just seems to sit at 0.8bar.
The second turbo isn't kicking in. This usually means your IACV isn't opening - the turbo is trying to spin up but can't flow any air, so #1 turbo does all the hard work.
By the time #1 turbo is producing 0.8bar at 5000rpm it's way beyond it's efficiency map and is overheating the air and overspeeding the turbine. Try not to do this too often.
Problem: I don't get a 2nd turbo rush, and as soon as I should it dumps all boost instead.
The IACV is opening but the second turbo isn't spinning. All the air boosted by #1 turbo now has an easy path back to the non-pressurised side of the turbos via a non-spinning #2 turbo. In effect, it all goes 'backwards' through #2 instead of going into the engine. You'll maybe see .2 or .3 of a bar of boost but that's about it.
This usually means the EGCV isn't opening. The turbo can't spin as it has no exhaust gas flow path. Although annoying, this won't do long-term damage as #1 turbo won't generate much boost because the engine loses a lot of power, so no overspeeding or overheating.
Problem: The second turbo comes in late.
It seems like, instead of 4000rpm, your second turbo cuts in at 5000rpm or later. Usually this is caused by no prespool taking place - your IACV opens, your EGCV opens, but the second turbo is sat on it's ass doing nothing. So, not only has it got to suddenly get up to speed, but it's fighting against #1's output trying to go backwards through it. Hence, the looong lag.
This is usually caused by the EGBV not opening. You may notice more than 0.8bar of boost being generated by #1 turbo because of this, as the EGBV also acts as #1's wastegate.
The usual cause of these problems is hose related. Either a vacuum hose has popped off or it's split. These hoses can get hard and brittle with age as they are subjected to continued engine bay heat cycling. It's usually a slightly split hose if the problem is intermittent, and a popped off or completely holed/severed hose if it's permanent.
To troubleshoot your problem, look at the pressure map of the system and trace the hoses/pipes between the suspect components. Check they are on and intact - removal of the hoses may be necessary to fully check for splits.
Edit I deleted the images off my personal webspace so I've uploaded them direct to the thread. Pressure map first then all the component locations.
For images and descriptions of the pipework