My car does the same.The only way is to install a connector pipe,x or H style between the 2 exhaust pipes.There was a discussion on this subject on another post recently.There is a natural tendency in the motor that causes this condition.In my own case I have learned to with it for now.
"Popping" on over-run is generally caused by excessively lean idle mixture or air being introduced into the exhaust system through a leak somewhere. Chances are the vacuum diaphragm in the diverter valve on your A.I.R. pump has failed; when they fail, the spool valve defaults to the open position, so air from the pump is dumped into the exhaust manifolds all the time, causing over-run "popping". When working normally, the diverter valve is designed to dump air out through the little "muffler" under sudden high-vacuum conditions (like over-run). When the diaphragm fails, it also causes a vacuum leak at the signal hose that feeds it from the carburetor, which causes even more lean mixture problems.