Until you sign a contract, you aren't "held" to work with any agent (be careful of creating verbal contracts though!). Be aware though, any agent you speak to until you sign a contract, isn't really held to client confidentiality. So if you look at one house with a realtor and say "It's great...but I can afford $X more, I'd like to see what THAT would get me" and then later happen to put an offer in on a home that he/or a friend represents, he can tell the client he has a contract with "He can pay more than what he's offering, put in a counter offer at $X more". Long story short, until you sign a contract, don't go into details about yourself because it CAN work against you later on.
As a former agent, I'd actually recommend *against* using the listing agent. They would LOVE to represent you (because then its 2x the commission off of one sale) but I have seen a lot of agents NOT represent one client or the other when put in that position.
It is possible to have commissions split between agents (ie, who shows you the house vs who writes the offer) but it can get ugly and you could be on the hook for a portion if the fight over money goes south. Any good agent you call should be asking right away if you already have an agent representing you (it's one of our guidelines in order to reduce the number of times one person has several agents "working" for him/her)