Is this pipe a copy of the G12CSR exhaust, say a model circa 1960? I don't know much about them but I think the left side crossed in front of the downtubes and clamped into a 'Y' on the right side. if the clamps are all tight and everything is properly aligned, the exhaust should be tight.
I've helped on a couple of 2-into-1 and a few 3-into-4-into-2 (Tridents), and the trick is to assemble everything loose, make sure the exhausts are home in their ports with the help of a leather or rubber mallet, and then tighten everything, front to back.
A slight mis-bend on a reproduction pipe can make the job a nightmare. I've had to tweak the pipes on 3 of my bikes to get everything to work, and those are all simple straight-run dual exhaust, but also all reproductions.
The 4th bike still has its original exhaust.
'73 Norton 850 Commando - 3030xx
'69 Norton Ranger 750 - P11/1289xx
'67 Matchless G15CS - G15CS/1235xx
'61 Matchless G80CS - 61/G80CS/41xx