[Backup question] 2 iphone with same icloud/appleid

That isn't true. Both of my iPhones had the same name for a while—I only changed the names so that I could tell them apart in settings—separate backups were made even when the names were identical. The backups are done by some unique identifier (maybe IMEI?), it doesn't seem to matter what the names of the devices are when listed in General > About > Name.

I backup 2 iPhones and 3 iPads all with the same AppleID to the same iCloud account. All you need to do is make sure each device has its own unique device name in the device’s general settings (settings - general - about - name).

Each iPhone will have its own backup in iCloud. You'll be able to tell the backups apart, when viewing them on your phones, because one of them will state "This iPhone"—see the included screenshot. I also have two iPhones using the same Apple ID and my backup screen shows separate backups for each phone. I've renamed my iPhones to better tell them apart when viewing screens that contain information about both phones.