Buyer: use same ID for 2 Metatrader's platform in the sam time? I did not try ... may be not.
Seller/Vendor: sell 2 or more IDs and use 1 or more buyer's IDs in same copy of metatrader? It is not a problem at all.
I am having many copies of Metatrader in elite section with RAS for example. One copy is having 4 charts with 4 EAs and the other 5 charts with RAS EAs (9 charts totally): every RAS EA is one ID selling + 1 RAS with timefilter. So, I have just 1 copy of Metatrader with 4 EAs and totally I am selling 5 very different IDs/signals. Free for elite members but just for example.
Same with buyers.
As to use same ID as a buyer simultaniously on 2 metatraders ... do not know sorry. I know that same people are trading at work and closing the trades at home

just because they are working ... it is not a problem of you are closing Metatrader for 1 or 2 hours just being sure that no one open trades will be closed in that 1-2 hour period.