So the first point to make is that it's a matter of fact and of record that you are a co-founder of both Startup 1 and Startup 2. Given that both ventures seem to be making good progress, that makes your resumé a positive value to each startup.
Secondly, though you don't explicitly state it, I can't believe that the two are competitors, so there wouldn't be an issue if (say) you were planning to be pursuing each half-time, by agreement with the respective teams and investors.
You're right that, all things being equal, investors like to see people 100% committed to their venture. But, of course, that is never going to be the case. We all have some kind of life outside of the day job, even if sometimes it doesn't seem that way. And sometimes the two will overlap. Investors know this. And, while they might not like it, they accept it - because it's reality.
So there's every possibility you can be all in on Startup 1 and still involved in Startup 2, and see both successfully funded. To maximize your chances of achieving that, your first step is to negotiate with both startups pre-funding.
My thought is, start by talking this through carefully with the team on Startup 2. Are they clear, and comfortable, that the first call on your time is Startup 1? What do they need from you now? What do they want from you during a funding round? What will they need from you post-funding? And what would it cost them - in time, money or any other way - if you had to cut all ties?
Next, you have a similar conversation with the team on Startup 1, only this time the question is, how do you formalize a personal commitment to Startup 2 that's acceptable? Look at different approaches. How would it work if (say) you wanted the ability to take out up to half a day each month? Or if you had to commit to only using time out-of-hours?
If you can come up with workable arrangements that everyone is happy with, then you have a credible starting position to offer to prospective investors. And you're also armed for a negotiation, if it turns out that this arrangement is actually preventing you getting invested: you're in a position to take a clear-sighted view of what would be the cost to you of being forced to exit Startup 2.
It's not exactly the same situation, but I was once in a startup team looking for substantial funding for a capital-intensive business. It was the norm in that sector for investors to insist on personal guarantees from the directors - they wanted everyone's houses on the line.
We were a credible team with a proven track record and a strong plan. And we agreed between ourselves that we would not offer personal guarantees, and that we would put this on the table up-front.
Plenty of prospective investors tried to push us on that. We never wavered, and of course that ended the conversation with some. But we got funding, on terms that were typical (i.e. there wasn't a cost to us of holding this position), and with no sense that it took us any longer to raise money.
Looking back, I think that the fact that we had worked through together issues such as, what motivated us individually and as a team, how would we stay focused on delivering results and so on gave us credibility. And holding the line on a decision we had come to together also showed that we had maturity as a team. So we looked like a better risk than a less tight-knit team. So who would get backing - the team more likely to succeed, or the team more willing to get caned if they failed? I'm not saying we didn't have to keep on restating our position and check contracts very carefully, but it worked for us.
Investors are (usually) reasonable people. And, if you're sufficiently transparent with your business partners that they are happy, and sufficiently objective about your own situation that you have a plan if you find that the situation is not just unusual but actually turning out to be a roadblock on investment, your situation may be to the benefit of both startups.