As Trevor has stated, the screw under the top lever if turned 1 turn, I say one turn because that screw has a tapered head like the top tang that it goes through. I have seen that screw sideways, half turn tight where the large taper is on the down side of the tang and 1 turn too much. This screw if the gun is turned up and the trigger guard removed, the end of that screw can be seen. The best thing to do when disassembling, is to take the trigger guard off and measure the distance from the trigger plate to the end of that screw. This way there is no mistake if the gun was shooting correct before.

Very seldom is having a two trigger gun with the back trigger not working correctly, it almost sounds like a safety problem.

I think taking the left lock off might show what is happening when you pull the rear trigger what is the trigger plate doing, it should have a little play but not enough for the rear trigger to trip the sear with the safety on..

I would do as Trevor stated and back the top lever screw out 1 turn and see what happens.