Im doing a homemade jobbie so i was just wondering if this was possible to dump some extra backpressure. Im very sure my current setup will choke the engine. But i dont want to go bigger on the turbos for cost reasons.

 

I don't really see why not. I can't imagine it would hurt anything anyway.

 

Running turbos smaller than needed is BAD. This runs them out of their effciency range and leads to compressor surge or a turbo huffing hot air into the engine.
High wheel speed + low air flow = surge (wheel stall, basically)

You're better off saving the money on the external gate and trying to upgrade at least the exhaust housings or wheel trim on the turbos. Adding another wastegate will do nothing but cost you money, they won't change the efficiency of the turbo.