I make a mild ale (3.5% or so) that I like to call Brown Trout Stout. It has a good bit of roasted malts in it, but also some good malty flavor (I use a fair amount of Vienna and crystal).
If I were you I would brew up a batch of a malty pale or amber ale (around 5 or 6%) and then you can test blend a couple beers to see whether its worth blending the entire batch. You will probably want to keep the hoppiness to a minimum for blending. Don’t necessarily just assume a 50/50 split will be the best, try a few different proportions.
Or you could do as you said and bottle/keg the second batch completely by itself and make black and tans right before pouring. The thin stout would surely float on top of a heavier batch.
If it’s a lost cause at least you have the second batch.