This is starting to get touchy because he's starting to feel like he is doing the CEO work. Because I'm the only engineer, I'm also in effect CTO, which leaves me less time to read/focus on the strategy and more time for him to do so. I still manage fundraising, legal, taxes, etc.
Is this normal/healthy, or will this inevitably lead to issues? Does anyone have a similar split?
If your customers/investor come to you because of your R&D, R&D head should control the company, earn most, and all other work should be done by other co-founders.
Otherwise, if they come to you because of your product strategy, then you should let that guy control the company, it's good for you at the end.
If none of you know what is the core competitiveness of your startup, you all should more concentrate on the product/service/marketing per se. Who take control is not important and you all will learn a lot.
Here's an interesting thought exercise: if one of you were suddenly unavailable (hit by a bus, won the lottery, etc) and the remaining person had to hire contractors to fill in that role, would the company continue? Is it the same company with the same vision? The CEO is the chief storyteller... the keeper of The Lore. In a very small company that does not yet have its own self-sustaining culture, replacing the head will result in a company which is something "other".
More practically, I'd advise you to think carefully about what you personally are trying to achieve. Want to remain in the weeds? You may be much happier transitioning to a more technically-focused role and handing off the strategy bits. Are you high-control and want to drive the ship, even if it means stepping away from the tech? Then execute like crazy in the big chair and truly own your role, but understand that this probably means recruiting technical / technical leadership help sooner rather than later.
That doesn't necessarily mean creating a strategy, but sometimes just approving one.
And like in your case, often a startup CEO gets burdened with technical, funding, sales, finance, legal, and so on. I personally resolved never to be a CEO & CTO again, because you're effectively a solo founder for a good portion of time - the non-CEO cofounder can't do a lot of things and development is often the bottleneck.
If so, stop worrying. If not, your problem isn't to do with job title and responsibility mismatch.