I'm sure it can be done, but I wouldn't suggest it the minute you introduce actors into the mix.
I've only worked on one feature so far, (as Dp) but I doubt we would have gotten anything done with a Director doing the Dp work.
It seems essential to me to have everyone focused on their particular job in order to be productive.
I had enough time to do my work with a few 5 minute breaks here and there in between light setups and a 30 minute lunch break.
Even then, on my breaks I'm thinking about my next setup, or explaining it to my grip so we can save 2-3 minutes when we're back on set.
I don't see how I could ever of had the time to direct as well without seriously compromising both jobs.
