The Firebox has the combo mic/instrument preamps so you can potentially run both guitars straight into it. You can set the device buffer down to 64 if it supports it and monitor everything live straight from the firebox since it has multiple outs. If you don't have another device that can power headphones then use a splitter on the single headphone out on the firebox. It likely won't get very loud.
Now if you want to keep a combo/instrument line free then run one guitar into the Boss and run that into a line in on the back of the Firebox. Then you can use the free combo/instrument preamp for a mic or whatever.
To get some decent sound out of a direct guitar you're going to need some amp sim and cab sim plugins. Look to this post for LePou amp sims and God's Cab IR library.
http://forum.cockos.com/showpost.php...6&postcount=17
Guitar -> Firebox combo/instrument -> Reaper Track1
Track1 FX -> LePou amp Sim -> Reaverb with Gods Cab IR
That's the basic gist.