Why do you want two of the same size turbos?
You could bolt 10 of them on there and they will still only produce the same amount of boost as just 1... Do your research on turbos before you fork out massive coin on turbo conversions...
And its asking someone if a turbo is too much for a particular engine is like asking a mechanic whats wrong with a car before he has even seen it...
Its not the size of the turbo(s) that's bolted to the manifold, its the amount of boost its forcing down the throat of the engine...
A Borg Warner S300 is a good sized turbo to begin with.... If you use up all the boost it can offer then you have a very strong engine...
Your car should be safe with 8psi and if you wanted to up the anti, then replace the head gaskets with a set of commetic head gaskets just to be safe and take the boost up to 12-15psi... It will handle more but you might throw a rod or melt a piston.
The tune is going to be no less the $800, but realistically more close to $1000.