Some agents have an exclusive submission policy regarding materials - i.e. you can't send your query or manuscript to another agent while they are reviewing it. I STRONGLY believe that this is an unfair policy. The only times I can think that an exclusive might be appropriate is if the agent is an absolutely perfect fit for you or they happen to be one of the biggest and best names in the business. Otherwise, my suggestion is to submit to multiple agents at a time.
Most agents are extremely busy and out of necessity are slow in responding to submissions. If you wait for a response from each of us before sending your project on to the next agent on your list you could be querying the same project for years and years.
My only caveat with multiple submissions is that you need to tell agents who have your partial or full manuscript when another agent has expressed interest in representation and give them at least a few days to get back to you.