It's not as simple as that. Both airlines have to agree to have the agreement.
By the way, there is something that seems to be getting lost here. Codeshare and Interline agreement are not the same thing. Codeshares are the alliances the airlines have such as Sky Team, One World and the Star Alliance. With these, any airline in the alliance can sell tickets across airlines.
An interline agreement simply means that the airlines agree to accept each other's tickets and baggage. So if you are on Delta, and the plane breaks down, they can book you a flight on American. But you cannot go to Delta and buy a ticket on American. Also, if a travel agent books a flight on say Delta from Dallas to Atlanta, then American from Atlanta to Chicago, then Delta can check your bags in all the way the Chicago and transfer them to American. But if something happens on Delta and you miss the AA flight, Delta is not responsible for that.
This also leads to something that many people don't understand about going between airlines when things don't go right. I keep seeing people saying that if the plane breaks down that by law you can just take your ticket to another airline and get on. Not true. Let's say you are on American and the flight cancels. So you go to Air Tran. These two airlines do not have an interline agreement. Air Tran is under no obligation to take your ticket, although they can. In reality if Air Tran takes the ticket, then AA would normally pay it out, but there is nothing to stop AA from telling Air Tran where to stick the ticket.
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