Would it be unethical to work in two different indie projects at the same time?

Just keep in mind that unless the asset in question is yours, or you are not under a legal contract about said asset that you created, you should not be crossing them between projects.

Ethics are difficult to pin down, beliefs vary by individual, and don't usually matter in this type of project.

More relevant is the legal side, which is society's baseline ethics.

You own what you create and can do with it what you like... initially.

If you signed any legal forms or agreements, perhaps an employment agreement or collaboration agreement, those can bind you. If you make assertions or grants to others when you contribute to those projects, including through email and clicked-on agreements, those can also be binding.

You might be legally allowed to share with both, or you might not. You might legally or contractually be required to notify them of a potential conflict of intrest.

Without knowing your details and agreements you have made, it is not really possible to give mch more solid advice than "maybe" and "talk to your business attourney."

Generally, the only time I've seen problems is when there's some kind of no-compete clause in effect. I've never participated in an indie project, but if they didn't explicitly have you sign a contract with a no-compete clause or something equivalent, you can do whatever you want.

If you're intentionally stealing code from one project to use on the other one, and you're not supposed to do that, then yeah, it's unethical. But if you've got clearance to do that, then it's fine.