Those on a newer release of E10 - does co-part now work for parts on the same job but on different levels of the bom/different assemblies - this does not work on 10.0.7.4 which we are on.
The reason I am asking is that you could make these ops quantity only and when creating/planning the job make them co-parts (done at the same time) - you would need to report a quantity for both ops but this could be done on the same end labour transaction.
There is the limitation with co-parts as above but you could work around it if you put the two parts at the same level on the method - from your original post this would seem to be irrelevant and I think the costs would all rollup ok for the highest level parent assembly - cost for both part a and part b, it is just that part a would not have part b costs