Any motorcycle mechanics here?

A lot of factors to look at here...
Compression and leakdown tests to assure good cylinder sealing, and similar #s on both sides.

Ignition? You basically have two seperate ignition systems on that, once one side is properly set and timed, you have to follow up with the other side.

Have you ohmed both coils, wires and plug caps? Points in good shape?

Both carbs set up the same? As in both have the same jets, and similar settings on mixture and needle height? Have they been synchronized properly? You have to first get both slides adjusted to the same vacuum, then even up both cables to pull the same.

A lot of times those old Amal concentrics end up with a lot of slide/bore wear, as well as end up with out of round bores from overtightening.

Chilton has a great book on BSA,Triumph and Norton. One book covering all the twins.

Just some food for thought.