I don't know of anything quite like what you're describing, but there are "half" or "twin" ropes, which are meant to be used as a pair. They're each thinner and

I don't know of anything quite like what you're describing, but there are "half" or "twin" ropes, which are meant to be used as a pair. They're each thinner and lighter than ropes rated as "single" ropes, but together are considered as strong.