I don't alter my limits whether I am alone or not. If you do something with a buddy that you wouldn't do on your own, then I suggest that you are too reliant on the other guy, which, to me, isn't right. I am assuming that if you are diving solo, you are an experienced diver.
My diving is virtually all open water, with some wreck penetration. As others have mentioned, where there is a greater risk of entanglement, I would be more inclined to "share the wreck" with someone, but it is unlikely that we would dive as a "buddy pair" in the traditional sense. (If you've ever met the dudes I dive "with", you would understand why I dive alone!)
Virtually all of my diving is in the 100' to 180' range, which I do alone, other than my boat-tender side-kick. I frequently do 100'+ night dives on a favourite wreck in Tobermory (Arabia) and it is probably my all-time favourite dive. Nothing like midnight on the bottom. Some nights, when there is a full moon and the water is clear, I don't use my light, as there is enough ambient light from the moon to (just barely) illuminate the wreck... once my eyes get used to it. But I still carry at least two, and usually three lights anyway!