From your first post inwardly I said to myself, "grab it".
I'm in a similar spot, I have a nice old black SL, bequeathed to me by an old friend. I located a nice 2 cam first version 50 Summicron, and it is a nice combo.
Since then I scratched an itch I had been putting off for a while. I picked up a nice R8, and also an R28-70 (ROM). Mistakenly I figured I could use the 28-70 on both the R8 and the SL, but was wrong, it will not fit the SL.
Then I found a nice R35/2, which is 3 cam, this at least fits both cameras.
Part of me says, stash the SL and 50 Summicron, get it out on the odd occasion, but just use the R8 for most use. And with the 28-70 it is indeed a convenient machine to use. I often have fun with just the 35/2 on it as well. Compared to the M6 and 35 Summicron I also have and use though it is huge, a beast, but I do like the SLR use. For travel (Europe again later this year) it's a no-brainer, M6 all the way, but for round home, having fun, the R8 is nice, real nice.
So in respect of your quandary, the SL2 would be a nice acquisition, but it comes with it's own built in headaches, mainly (like my situation) the difficulty in compatibility with ROM/non-ROM lenses. I've addressed that in my own mind, buy only 3 cam lenses, and put up with the 28-70 being R8 only (or sell it I suppose).
Whatever you do, you'll have fun.