If there is only one sensor that monitors both exhaust manifolds together and then you cut one manifold off from it to where it is only reading the O2 in one manifold instead of both, you're going to have problems unless you make it where you have 2 O2 sensors (one for each side). Don't know how you would do this and make it work, but that is what you would have to do in order for the engine to work right. The only other solution is to leave the two pipes joining at an H pipe and have the O2 sensor there so that the sensor will be reading the right levels.

Richard