Jeremy,
I have both an HP Laserjet and an Epson 3800, but on a PC, not a Mac. However, I did once experience a problem that seems similar to yours, so here's another possible thing to try (but no guarantee): In my case, I couldn't get my Epson and my HP both to work if they were both connected via USB, but when I moved one of them to a Firewire port, everything worked. It seemed to be a case that there was some conflict between the HP driver and the Epson utility.
So maybe, maybe, if you can put one of them on a different kind of port, you might get things to work (and be sure to throw salt over your shoulder while trying this).
Good luck!