A few days ago, I posted about simultaneous submissions for publications.
But what about literary agents? There are a number of agents who also specify this.
This I wouldn't be too worried about, because unless you find yourself in the position of having two or more agents fighting over you, chances are they'll understand that you queried multiple agents with your query letter.
However . . . if you do find yourself in that position, make it clear to both of them that there are multiple offers on your work and you will give one agent a specified amount of time to look over and read your novel. Then, if one turns it down, go to the other one.
Keep in mind, the simultaneous submissions only come into play when an agent wants to read the entire novel. If you have two or more agents reading the first twenty or fifty pages, that's okay. You may want to mention to them that other agents are also considering, so this could heighten the need to represent you. Not a bad place, if you ask me.