There is an approved Boeing procedure to board/service/fuel an aircraft with an engine running. I did this once in Finland when the GPU failed and we arrived in a 400 without APU. Pax off first, with no1 shutdown, crossbleed no1again and shutdown 2, then, baggage, refuel and crossbleed no 2 and shutdown no1 for pax boarding, then crossbleed 2 again and home!
Totally dodgy but approved on a very one off type basis. If you are looking at doing this to save time or money or some LCC type mental idea its a non starter.
Good Luck!