My full manuscript is out to two agents, they've both had them since early July 2009.
Can I query them with another book? Or should I wait to hear back on the first one?
What if I never hear back (meaning they've passed on my novel)? What's a reasonable amount of time to wait before querying them with a new project? Six months? A Year?
Early July 2009 is pretty recent, so I wouldn't be feeling impatient just yet.
I would not query an agent considering your work about a second work until you've heard back on the first book. If you haven't heard back within ninety days, a friendly follow-up is fine. Then follow-up every ninety days until you hear something, unless, like me, your agent does something pro-active on a blog or otherwise to keep you informed as the status of the submissions under consideration.
Once an agent has passed on something of yours, I'd give it a couple of months before querying again. Let him or her clear his or her palette a bit.