How much butter is in your frosting? Because you may not need to freeze it to get it hard. I refrigerate mine for a bit and do the smoothing thing on it...but my BC uses just butter and gets pretty firm just in the fridge. I don't use shortening but I imagine that may not get as firm in the refrigerator...but then you really don't need it to be all that firm if all you're doing is frosting over a crumb coat.
If you just want it to crust leave it out and it will crust on the outside but still be soft on the inside. That's the best time to use either the Viva towel method for smoothing or the roller method for smoothing.
i did a half and half cake like you're talking about. It was two 9" squares put together. If you're putting the pieces together you will have to make sure to level each cake so they will look like one. Are you coating in just one frosting or doing half and half on that as well. Here's my cake and how I approached that:
(please forgive the lousy photo...and the fact that this was one of my earlier endeavors! ) http://cakecentral.com/gallery/1836000/sheet-cake
This was a one-layer cake. Had I to do this again I would have stacked them two layers with a filling to make it taller and cleaner looking.
Here's one I did recently only with a round cake. You can see how I approached the half and half issue with this one: