There is a legendary example of this: Jack Dorsey being the CEO of both Twitter and Square simultaneously. Yes, it is possible to hold two jobs. It's even possible to be the CEO of two billion-dollar companies. It's far from easy, but it's possible with an extreme dose of discipline and time management.

I heard Jack personally tell the story of how he managed his time at a fireside chat he did in New York. The closest I can get to an online article that talks about it is this: http://money.cnn.com/2011/11/13/technology/dorsey_techonomy/

Take a look at the way he splits the days of his week into themes. Each of his 6 working days has a purpose.

Monday: Management meetings and "running the company" work
Tuesday: Product development
Wednesday: Marketing, communications and growth
Thursday: Developers and partnerships
Friday: The company and its culture
Saturday: (no work)
Sunday: Big picture strategy

But he then goes next level and splits his time between two companies by alternating mornings and afternoons in each office. The article mentions him working 16 hour days – 8 hrs at each company – but when Jack told the story himself, he said he was splitting mornings and afternoons between the companies.

Monday Morning = Twitter
Monday Afternoon = Square
Tuesday Morning = Square
Tuesday Afternoon = Twitter
Wednesday Morning = Twitter
etc

This might be an approach you can think of taking. It means you make it clear what your schedule is, when you will be working on different aspects of each job, and then be incredibly disciplined about sticking to the schedule so other people can accommodate and work with you.