With commissions, people can just put out their idea and multiple artists can place a bid. With requests, people go to the artist with their idea. Also, members can make a request without putting any EVs in the idea, so you can hold off on working on it until it has all the required EVs.
As for if an artist can draw a request from another artist's page, I'm not staff and not sure of what any restrictions would be for an artist doing that here, but I'm sure the request and any EVs that were put in it would remain on the requested artist's page.
While comments on an artist's request are only separated by their EV tier list (50 EVs, 100 EVs, 150 EVs etc), you can put the requester's name in your comment to indicate who you're talking to. If they put multiple requests in the same EV tier you can mention the idea as well.