I have heard if you mix favorite mix cake with box of pound cake mix and all ingredients listed for both you get  a delicious cake better for carving  anyone else heard this?   I thought of doing this then dropping dollops of chocolate cake mix and marbling   think it will work?

yes i think it will --

on the marbling -- you just want to be careful that the textures are not too far off -- often marble cake has some of the vanilla batter removed and chocolate added to it so it will be a bit different texture -- you should be fine with this