engine are connected. If you don't think so, why do you have to cap both sides of the intake tubing when doing a boost leak test? There is the balance tube on the intake manifold. There is your proof. That balances both side of the engine manifold pressure. What do you think happens when you boost on one side of the engine and not the other? That boost is going to leak out the other side of the engine through the compressor side of the non boosting turbo because it is connected. There is no absolute wall separating the two . If you don't think so , try capping just one side of the intake piping and hook up a compressor to the capped side. See where the air escapes. You will not be able to build pressure.