install a backup onto two different ipads

Your backup can be restored to multiple devices. While locally stored data will then be independent for each device, assuming that each device has an active network connection to the internet, any files that are synchronised via iCloud will be the same for each device.

Settings > [Your Name / AppleID] > iCloud - enable Apps and Apple Services that you wish to synchronise

All Apps that you have purchased using your AppleID can be installed and used on all devices that use the same AppleID - and for Apps that permit sharing using Apple's Family Sharing feature, used by up to six other members of the family group.

yes, you can restore a backup on many devices, though when the restore is done it does not mean they become a clone from there on, so they will from there on become 2 independent devices

unlikely, all apps are tied to the appleID never the physical device, and the whole point of appleID is that you use the same one on all apple devices you use, from phone to iPad to mac to..