Can I back up my iPad from an earlier back up than I used for my iPhone

iCloud does not keep archives of past backups. It only ever has one backup per uniquely named device and that is always the most recent backup. iTunes also does not, by default, keep archives of past backups. Each new backup over-writes the previous one.

If by "backup your iPad", you mean set up and restore the backup to the iPad that was made from the iPhone, then yes.

You can use the iPhone backup, to set up the iPad.

If the iPhone was running an iOS version that is newer to the one on the iPad, you will need to update the iPad, before you can restore the update.

See here: Restoring an iPhone, iPad, or iPod touch backup from iCloud or from a computer when a later version of iOS or iPadOS is required - Apple Support

You can restore whatever backup is in iCloud to the iPad, this will not affect the iPhone at all.

As mentioned, there is only one backup in iCloud, the latest one.

If the iPhone has not backed up since January 17, then that's the one that will abvailable in icloud.

Otherwise, the newer backup overwrites the older one.