Slitherine

when working out shooting if you have a 2 bg 1 of musket and the other arithery do you add the dice to hit if they are shooting at the same bg. only i dont see how a artilery unit on there own can do enough damage to force a cohesion test un less you have quite a few

Yes. All hits are totalled against the target and then CT and death rolls done accordingly. You may have to roll seperate die for the shooters if POA's differ.
Also bear in mind when doing CT the -1 for under artillery fire counts even if the artillery did not actually contribute any hits.
And just in case, like me, you do earlier periods then shooting bases with old fashioned bow/crossbow must be outnumbered by gunpowder shooting bases at the same target for the -2 on death rolls not to count. Again, who actually scores the hits is irrelevant.

Also, artillery don't need to cause CTs to damage the enemy. They cause a trickle of base losses that can be fatal if a BG hangs about in front of them for too long.