I've done it for exactly two months, I am a remote contractor and had started a new contract when I already had one running, did not think it was something I could've kept doing for long but I'm happy I tried it.
I was working two full time jobs in two different startups. Scheduling meetings between different parties was not an issue at all, I did scrum meetings in the morning in the first contract which happened every other day on the same hour, and I would make sure to schedule the other meetings in the other contract towards the end of the day. What I did notice is that, although I could pull it off if I really wanted to and needed both jobs, and although I love working, I had to trade off creativity in my work, as in I couldn't pull an all-nighter working on new idea for an aggregation framework or try to get ahead and impress anymore, and only felt like working to get the work done and get through the day. I believe that the quality of my work remained the same, but I lost all the fun in it, and strongly felt that my time wasn't my own anymore. Needless to say, it kills all your time and [depending on the nature of your job] you gravitate towards robot-like behaviour and lifestyle.
Wouldn't do it again, I can afford to make less, but happy I got a glimpse of what was humanly possible for me to do, even if it's only for a span of two months!