[jd_illa]

check with a lawyer, but I suspect the answer is no. Visa rules tend to be very restrictive in the US and as noted on this thread, those restrictions usually say you can only work for your sponsoring employer and no third party. I don't know if it's possible to amend a L-1 visa so you're working part-time at two different companies (FAANG + your startup). Perhaps that's not something you want to do in any case.

There is a question of risk allocation: if you hold unvested equity, you probably aren't paying taxes on it until it vests or you exercise it (depending on the equity), so would USCIS (who issues visas) know if you are working part-time if there's no cash component? I don't know. But the risk is really high for you and the startup bc if the government does find out that you're violating your visa rules, they can deport you (potentially permanently) and also subject the startup to consequences.

If you are really excited about the startup, it's probably not a bad idea to check with an immigration lawyer about your options (this is something that they could probably answer quickly in a free consult).