A bride wants a three tier square cake that is covered in fondant - half white, half red.  But she doesn't want it just split in half (that would be too easy!)  but rather wants the color to split the cake on the diagonal.  I've never done a two tone fondant cake, and was wondering whether the better way to approach this, instead of trying to cover half in white fondant the other in red, is to use my airbrush, and mask off the areas  that are to remain white and airbrush the red areas.

One complicating factor is that I will need to refrigerate the cake after it is covered in fondant - the fillings are perishable.

The other option I was considering was covering each square cake entirely in white fondant, and then using red modeling chocolate "panels" to create the "half and half" look she's going for.

Any thoughts what the best approach would be?

IMHO I think you should go with the second option, simply because airbrushing half red, you will need to go over it a few times and will most likely result in the guests mouths turning red.

Personally, I would use red fondant instead of airbrushing since you must refrigerate.  This may not be a problem, but I would be afraid the airbrush color would sweat and run when removed from the cooler.  Since you are using a nice gold trim between the two colors, it should not be difficult to apply one color fondant, then apply the second, overlapping, then trim the two pieces at the same time so the fit is perfect (hope that makes sense).  It will be forgiving because your trim will cover the union anyway.