Can I record with 2 usb mics in audacity once I convert them in to an aggregate mic source?
Theoretically “yes”, though it is a bit of a hack because the mics are not actually synchronized. I did manage to get this going when a friend lent me a couple of USB mics, and it worked quite well. I have since returned the mics so I can’t give detailed instructions. Hopefully you will be able to get it working and post exactly how you did it. 
Record them on two different computers. Then it’s a snap to resolve any timing problems with Effect > Change Speed.
If you use the Aggregate Device trick, I believe the Mac will smash both sounds together into one channel. If there is any timing difference between the two microphones or you have a long show, the two will drift off sync and there’s zero possibility of getting it back because audacity can’t split voices, instruments, or sounds apart from a mixed show.
The show is toast.
If you plan on a two-person podcast with both people in the same room, it’s far worse because each microphone will pick up the “wrong voice” a little and timing differences will make them gargle, shiver, echo and then finally turn into two copies of the same person.
It’s a time bomb, too. If you do get it to work perfectly, you may assume everybody can do that—and they can’t. You can’t either. You will hit two USB microphones that don’t work at all. This is OK if you’re doing home recording and Tuesday you’ll be doing something else, but if you become rich and famous, this will eventually haunt you.
Or better yet post a YouTube instructional video how to do it and find out how many people can’t.
USB microphones are aggressively one-off, home recording. The limit for reliable recording is one.
Koz
By “Sync” we’re not talking about having one person further from the microphone than the other or interrupting each other. Each USB microphone has a digital clock-timer inside. That’s what makes the 44100 or 48000 sampling rate. Unless you paid an eye-watering amount for the microphones, they have cheap clocks inside.
One microphone runs at 44100.1 and the other runs at 44099.9, for example.
When you set up Aggregate Device, did you notice it asked you which microphone you wanted to be the master? That will be the “right one” and the other will drift away.
Koz
so is recording on 2 different computers the best solution or is there a device that can sync them together in some way?
a device that can sync them together in some way?
I don’t know of one. As long as you keep the two sound streams separate, you have a fighting chance of fixing drift issues later.
You can throw money. There are sound recorders designed to be perfect and match video recorders and each other with little or no sync drift, but they’re not $98 American.
Borrowing a second computer and installing Free Audacity (and check it ahead of time) is the best bet. This can have its own problems. Count the forum postings from people with computer recording troubles. Now you will have two.
I can be Debbie Downer forever, but if it’s not an important show, you should totally try Aggregate and see what happens. If you have a short podcast, you may get away with it and nobody has to know what you did.
Koz
It can do, or you can keep them on separate tracks. It depends how you set up the aggregate device.
When I tried it, each of the USB mics appeared as a stereo (?) USB device. I created a 4 channel aggregate device and was able to record 4 channels with one mic in the first two tracks and the other mic in tracks 3 and 4.
As I said, it worked pretty well in my brief test, though I’m not sure that I’d want to trust it for a major recording project. Your mileage may vary.