I have to make a red velvet cake and a chocolate cake. The red velvet is going to be in the shape of a cross and the chocolate is a sheet cake. Can I cover these in white chocolate ganache and then ice them with white buttercream icicng? Please let me know.

I've iced with buttercream and spread dark chocolate ganache over that before. I've never put bc over the ganache.

What about white chocolate bc? I use Indydebi's bc recipe and add 6 ounces of the Baker's White chocolate squares (the white 'premier' chips that are made by Hershey's, etc. are nothing more than candy melts...NO chocolate in them) the Baker's squares DO have cocoa butter I melt the w. cho. squares and cool to room temperature and mix in with the bc. You will also have to add extra milk because the chocolate thickens the buttercream. I just add about a teaspoon at a time until I have the consistency that I want.
This would be a good alternative to using white chocoalte ganache with bc, practically the same thing just less trouble.