If an agent doesn't specify that they don't accept simultaneous submission, then can I assume that they do? I have my novel with one agent now, but would prefer to have it with more if possible.
I always suggest submitting your manuscript or proposal or query letter to as many agents as possible--and to do so simultaneously. It is hard to land an agent, and hard to land a book deal, and it is also a very subjective and slow-moving business. If you submit to one at a time, or just a few at a time, you can be waiting years to further your career. I would always err on the side of a more aggressive approach. Even if three agents like your work, all three can still reject it when reading more. Don't wait on anyone--unless a bonafide agent takes a genuine interest in your work and is responding rapidly and requests a limited time window, say a few weeks, of exclusivity.
I speak to this topic at length in my book HOW TO LAND (AND KEEP) A LITERARY AGENT, which I give away for free. I suggest you read it. www.landaliteraryagent.com