USCIS recognizes that it can be really tough to get joint assets after arrival on a K1 visa., without green card or at least EAD card. So, no worries - just submit what you have. We (and a lot of others as well) sent in just marriage license, despite the fact my name was already on our AT&T bill, I was added to my husband's bank account, utilities account, house insurance and car ins (I got my first DL right after arrival , valid til my I-94 expiration date). No interview, no RFE for any joint assets.
Actually, we had an RFE, but due to looooong I-485 processing time (15 months) - they just wanted the latest Tax Return and some vaccinations were already due. That's it. But we were never asked for any joint assets during AOS, although we already had plenty.
Yes there is no requirement from USCIS or NVC or State Department that the applicant and petitioner have a joint bank account. As mentioned, it can help prove a bonafide marriage but there is other "evidence" that can be used to help prove that as well if needed!
