I think that a simple way to do this would be using Soundflower and Soundflowerbed.
I can’t see how to make the USB mics inputs to Soundflower and then pipe them into a Java.
It sounds like you have Soundflower installed already. Soundflowerbed is found in the same disk image as Soundflower and is a menubar application. It lets you route sound between applications which don’t have controls built in for selecting sound devices. Install that from the disk image and click it to run.
All of the following will be using my Echo Audiofire 4 but in principle should work on any audio device.
Using Soundflowerbed
Open Soundflower and tick the audio device you want to use under Soundflower (16ch). As I’m a new user I can’t post images but they are linked below. If I get the bounty then I will edit the post to include the images inline.
From here you would use Soundflower (16ch) as your audio input device in Java sound.
Creating an aggregate audio device
An alternative way to solve this if that didn’t work is to create an aggregate device. Open Applications > Utilities > Audio Midi Setup and click the plus sign to create a new aggregate device.
Tick the device that you want to aggregate. You only want your USBMIC (As I’m a new SO user I can only post two images per answer so the next two are linked here).
The key part which may be giving you trouble is the clock on the device. If you select the Mac as the clock source then that may be more stable.
If this still doesn’t work then you could try adding the Mac built-in audio to the aggregate device and making it the master clock by right clicking on the device you want to be the master.
Other options
Finally, I haven’t used this before but Pulse Audio (Google it, I can’t insert more links in this post) might be a possible solution for mixing your audio streams together. It looks quite heavyweight though.