Most of the people I buy from pack cels the way you do. So far, they've all made it safe and sound. 

When I ship cels, I also use two USPS priority mail boxes, but I assemble them, with paper in between. I place the cel in a USPS envelope and either tape it to the inside of the inner box or I tape it to a piece of cardboard which is placed into the inner box and surrounded with paper. 

I have shipped about a hundred such packages around the world and, to my knowledge, all but one have arrived safely. The one in question had a fist-sized hole punched through both layers of boxes and paper. The cel and the cardboard it was taped to were bent. It took a lot of effort to destroy such packaging, but it happens. It depends on the USPS and which route they're using. This particular package went south, via Atlanta. The only package I've received damaged came from the same place, also through Atlanta.

While my own preference is to assemble the boxes, I have shipped them flat when a cel was too large to fit the assembled smaller box. I've not heard of any problems from the people I shipped them to and, as previously stated, most people I but from ship this way.

-Mark