Personally, I wouldn't but yes it is possible as you said - Steve Jobs did it and Jack Dorsey does it.
However, this doesn't mean that you are Steve Jobs/Jack Dorsey and running two startups at once is hard work. Especially when, most people can't even run one!
There are tons of people's opinions regarding this for instance:
http://tech.li/2012/02/ceo-sundays-running-two-startups-is-m... - Argues its possible
http://learntoduck.com/startups/two-startups/ - Argues its not
FWIW: I'm not putting Steve Jobs or Jack Dorsey in the same boat, I'm just highlighting someone else who currently does it. (Nor am I discreditting Jack Dorsey by saying this - Square and Twitter are both great startups)