b vent

I am wondering if there is a clearance requirement from one b vent to another. I saw an older home today with two wall furnaces. The b vents run parallel to one another and terminate right next to one another. Clearance to combustibles is fine and the vents are properly terminated above the roof. While the install appears odd I can not find any reference that it is improper. Ideas?
Thanks for your time.

Do not think it matters .

Do you have pictures of the double termination?

no photos. I dont think it matter as well just never saw the configuration and wanted another opinion in case I was missing something.

Clearance is to combustibles. Is either B vent combustible? No.

I was thinking more along the lines of leak potential at the cap area since the pipes “terminate right next to each other”.