Hey Nate, I'd say brace the hell out of it, and you'll be fine. I'd follow @theloneisobar's suggestion and add the horizontal pallet on top. I would even add an third vertical one, connecting the two on the sides. You could place it half-way in the middle, making it look like an H from above. That way it would add maximum support. Then I'd add the cross-braces in ALL corners: along the top, and along the middle pallet, top, bottom, and middle, on both sides. (I hope this makes sense without a visual. If not, let me know and I'll draw you one.)
Sure, this way you'd lose the storage area underneath the IBC, but honestly, I'd second @josegomez1 in this regard: better safe than sorry!
One of the things I learned here in Mexico is that it's okay to use shoddy material (though your pallets don't seem that shoddy), as long as you use enough of it. One time I helped some friends build an outdoor kitchen, and my German mind was screaming inside me that we're all going to die if this flimsy building collapses... In the end there were half a dozen of us climbing around on the structure, putting the roof on, and it was stable as a monolith.