I have actually seen this done before, although I haven't tried it myself.
I would make a 2 sheet cake the regular way..
THen you get icecream that comes in box form. Keep it out in room temperature for about 10 min.. depending on how hot it is that day. That way you can more easily slice it. Then you cut the box in half and layer the icecream on top of the layer cake. You might need more than one box of icecream depending on how big you want the cake to be.
Then you just place the second sheet cake ontop and place in the freezer. Also, remember to have the cake fully cooled before placing the icecream ontop.
When the icecream fully sets, you can ice the cake and decorate any way you would like.
I hope that helps.