I am in this position and would add a couple of points:
1. The ammunition alloawances are individual. I.e. if you both have allowances of 200, then that meams you may have 400 rounds in total accordong to the police.
2. If anyone is disappointed with the backlogs and inefficiency of police firearms departments, they might condiser not wasting their time with pedantic enquiries given that they are not even qualified to give aithoritative answers anyway. There seems to be a large number of people who think it is acceptable to waste police time with unnecessary enquiries about all sorts of.things from whether they can shoot A species with X rifle, can they zero on Y ground, what if they want to shoot in another county, etcetc. The.licensing system exidts for tow reasons - to ensure you're not a danger to society and yourself, and to ensure criminals can't get hold of guns. Anything else is sheer pedantry and creating work which obstructs and undermines the system.
3. If you really want to know the answer, you consult an expert solicitor and pay for it.
4..It is very hard to imagine a circumstance where anybody really cares. In the extreme case, transfer some rounds between yourselves. When I want to go shooting, I do not waste time getting hold of the other holder (resident elsewhere) and engage in an accounting exercise where he transfers 5 rounds of .22 to me, and then after shooting I either replace them in the box (having scrined individual serial numbers on each round to avoid inadvertent unlicensed transfers), nor transfer the tjree remaining rounds back. During covid, I estimate about 6 to 12 such transfers occurred each day. It is absurd and it makes no difference to anyone.
5. If one is strict about this, then the estate rifle clause, RFDs when testing a rifle, instructors and perhaps rifle clubs(although i don't know) all have the same problem, but in some cases the recipient has no FAC to enter the transfer. Given that the activity is legal, it would indicate that pedantic ammo accounting is not intended.