It depends how much effort your startup requires. It also depend on the intellectual effort required by the other part job. If it's a manual job, you will be able to plan what you do the next time you work on your startup. You'll make better decision and you might even become more efficient than some working full time for its startup.

If the other part job his intellectual, then it will be much harder, especially if you can't well split activities.

Finally what is really important is how much perseverance, motivation and will you have. The other factor will just influence the amount of difficulty you have to overcome.