If you're wanting a combo list to reflect what's chosen in some other combo, that's typically known as cascading combos, which you could research to see how to adapt for your needs. I've never seen anyone asking how to do that on different forms though, and I have a hard time following your explanation. Maybe you want the value in the "one" side combo to be a value that's related to what has been chosen in the "many" side and the goal has nothing to do with filtering a list. I'd advise to forget that and just do things in the normal fashion as I'd describe that as the tail wagging the dog. I also suspect your tables are not properly normalized if you have designed fields where each value of an attribute has its own field (client1, client2, etc.).
Unless a moderator moved your post, you seem to have started your own thread here. Does that indicate that you copied your post from another forum?
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.