I did try to go to the copycat site you linked to--I waited forever and it just did not load!! That is the kind of site that might have what I seek--perhaps I will try it on a different computer later. My laptop is behaving strangely at the moment
The recipe you linked to is one I have looked at. However, they make it in a tube pan, and the Starbucks one is a loaf pan shape. So I am immediately suspicious Also, I don't think the Starbucks one has lemon in it--I think it is simply golden cake, if you KWIM.
As you say, starting with two cake mixes is a cheap and simple first try. If I get the single mixes in the flat package, not the box, it will make a single layer, I think (ie boxes make two layers, 24 cupcakes, etc) so I can get two of those.
I have always liked marble cake myself so it is funny that the kids like it too. Apparently they have had it at camp too, so now I know it is probably the marble bit they like, not just the pound /loaf bit. I have never made a pound cake, and never made a marble cake but I guess it is time to get out of my rut of my usual cakes (applesauce spice cake is the family fave) and try something new.
Thanks for you help!