I have two iPhones, when I back up one of them I have multiple dates of files when this as done. When I back up the other one there is only one file and that is the last date backed up, it appears to write over the same file, how I get back to another backup up file?
By default iTunes doesn't work in the way you're expecting, and with good reason. iOS backups take quite a bit of space and most users would run our of storage space pretty quickly if each iOS backup was being kept (especially with multiple iOS devices).
While you can get around this default behaviour, it's quite a manual process and would require doing it each time you backup your iPhone.
As for your statement:
I have two iPhones, when I back up one of them I have multiple dates of files when this as done.
It's not clear exactly what is happening and how, but your best bet is to make sure you have a backup regime in place. Assuming you're on a Mac, then using Time Machine is a good example of this (but there are many options available - both for Mac and PC).
Once you have a backup regime in place, your backups will include your iPhone backups as well (unless you've specifically excluded them). In this way, if/when you need to go back to an older iPhone backup, you can.
As mentioned above iTunes maintains only one backup per device. Every time you backup your device, your last backup is overwritten. You can choose to 'Archive' a backup in iTunes’ Preferences - Devices tab, right click a backup and select archive. This won’t overwrite the backups, but still each backup takes up a LOT of space.
It does not have to take up a lot of space. I work with the team behind iMazing, and it makes a 'shallow' copy of the current backups, so that the next time you back up, you will still have access to your previous backups, and only the files which have changed will take up disk space.