I'm a serial founder, and am starting two tiny efforts at the moment in separate companies with a few people I know while continuing to coach startups (which is fun, if not highly profitable). The ideas are mine, but I'm a lousy coder and want great software, and don't have a lot of money to invest, so it's a joint effort with a bit of skin in the game by everyone in the form of sweat equity. Not equally a joint effort, though.
Deciding who and what a founder is at the beginning of a business is critical. Sometimes it's clear--the business is a going concern, you've invested your time and money, taken all of the risk, and hire a few people who don't take those risks or make those investments, and you're the founder and they are employees. You might give them stock options, or exchange stock for intellectual property, but that doesn't imply any particular founder status.
Founder status matters and is an earned and fact-based status: it's the person or people who start the company, and take the risk and responsibility of funding it and keeping the doors open. It comes with a set of risks, obligations, responsibilities, liabilities, and privileges.
The downsides are significant, but the upside can be huge, so you take the good with the bad. Down the road it shows you've had the tenacity, dedication, and commitment to launch something successful, which helps when you go to start your next thing. Investors like to invest in people with a track record.
When you start something with someone else, it's likely different people will follow through with their roles at different levels, and even more likely that a single person will take on more responsibility and risk than the others. That's not always the case, but it's the likelihood. It can be a lot of fun having co-founders, but not all founding is equal.
One person will take the responsibility of raising the money, managing investor relationships, legal, accounting, hiring, managing, etc, and the investors will identify that person as the steward of the investment, while the other co-founders are working on their parts of the business. Usually that person takes the CEO role, but I've seen some companies where the co-founders really share responsibilities evenly. I don't believe that's common though.
One thing you might consider is not having co-founders. Frequently the roles they will play at the beginning can be filled by others with just employee status. The upside is that you don't have any potential for co-founder conflict. The downside is you get less commitment of time and resources by employees than you do co-founders.
Finally, you should have roles, rights, obligations, compensation, ownership, timelines, expectations, values, personal goals, company goals, and responsibilities defined up front. Not that they won't change, but it helps when you avoid assumptions about a range of things up front and get agreement on areas that are likely murky.
Some advice: if you have them, check in with your co-founders regularly. Maintain the relationship by regularly sharing how you feel and perceive things. Don't get frustrated or judgmental; it's likely everyone's working hard. Make sure your goals and values are aligned, and nurture the relationships.
Starting a business is hard work and can be very isolating and emotionally difficult. Even if you have co-founders, it's likely you can't share everything with them. So find other founders and form a support group, or hire an executive coach. I hired a great executive coach/life coach, and it made a huge difference for me, especially when I was making mistakes and feeling lousy about it.
You don't have to go through this stuff alone, and sharing your challenges and feelings with the appropriate audience (not your spouse, and definitely not your employees or investors) can be incredibly helpful. I'm looking forward to the new startup, and am taking my own advice.