First, I think that might be a little tight to get those things in there, and maybe you might want to consider a wide body trailer... Even if it works now, it sounds like all it would take is a little bigger ATV tires and it wouldn't.
Anyway, you can add ancor points where you would need to strap both down at 4 corners, and a wider garage would give you a bit more breathing room and keep the walls from getting beat up.
Myself, my quads are right at 4' wide, and the exterior of the trailer is 8', so I have to turn the 600+ pound machines sideways (not fun).
Second, if this is going to be your first toy hauler, I'd highly suggest going to a big RV show to look at all the available floor plans and quality levels, then start out used.
Just about everybody learns from their first one what they really want... New RV's depreciate hugely when they roll off the dealer's lot, so you would really take a beating if you start out with a new one and then trade up in a couple years.
Dodge Ram dually laramie 4x4 w/jake, B&W, Brakesmart.
Kit Patio Hauler 394F toyhauler 5th wheel
Camo 680 Rincon, Green 500 Foreman, Blk twincam Roadking