Go with the company you're most passionate about and you'll make a living or more.

I wouldn't do B just because you think there's some pot of gold at the end of the rainbow. Your health and sanity isn't worth that trek, especially if you having some else you love.

Quite a predicament. I have actually experienced a similar situation. To fully answer your questions I would need to know more about each business, such as the revenue each entity is generating, and what your role is in each.

If you're profitable, and can afford to hire a replacement for yourself, and act as a consultant and/or split your time for company A, you may be able to spend more time with B. The other trouble you are describing is non alignment of partners. This is potentially a bigger problem in the long run, especially once you generate some success. If fundamentally you disagree with both, and can't align with one to vote your position, then you should probably leave B, no matter what. The stress level will only increase as time goes on, especially once real money is involved.

I have been fortunate enough to have alignment with my two partners. Not to say there aren't any grey hairs from the epic battles, or that no one got hurt in the late night bun fights.

Hey, you've phrased the question in such a way that suggests that you already know the answer. You're just looking for some confirmation.

The right answer is to withdraw from company B gracefully.

The good news is that you have a clear vision about your commitment to company A and that company B is doing well so withdrawing may have minimal negative impact.

It may be a good time to plot through how to manage that graceful exit. You may even find that it generates goodwill with your co-founders in Company B.

What you're experiencing is pretty typical. The process of focus involves a fair amount of defocusing -- we just don't talk about it much.

I've been through the growing pains of three startups, in each case gradually withdrawing from one to pick up the next. Let me know if there's anything I can do to help. Cheers!

Presume you have equity in B, but did you invest capital? If you don't see eye to eye the other partners will be just as happy to have you take a back seat role. Keep your equity or lessen it and take a cut of deals you bring to the table but let them run the day to day.

Sometimes you have to walk away from one and focus on the other to really appreciate and recharge the thought process for Company B.

Real life scenario is Twitter and Square....Jack Dorsey was part of the Twitter Team and then left to focus on Square and start really building it while Twitter seem to lose momentum only for Jack to return and now is CEO of both companies spending 50% of his time each day on each company.

Twitter just announced IPO and I am sure Square will not be far behind in the next 12-24 mos.

You have provided a lot more information about company B than you have for A. All I know at this point is that you are passionate about A, it's your baby and you have no partners to deal with.
I would like to dig deeper into the source of your stress in company B since it would appear to be doing better than A ( Just a guess because you have not spoken to the profitability or success of A)
Working in a partnership means that your ideas and suggestions will be challenged and your vision may not always be supported. It requires you to develop the skills of influence and humility; Influence to get the cooperation of others for your vision and humility to follow where others have the better idea. If you are all strong visionaries, it's going to be a hard balance to achieve, but one that is well worth it especially where your business is a winner.
Again, I have more questions for you than answers at this point l, especially since I don't have the full picture; but if my guesses are right, there might be more to be accomplished with A AND B than you might think my friend.