Using multiple iPads on 1apple ID

Sign-in to iCloud with the same AppleID - then enable iCloud for each service that you wish the synchronise:

Settings > [Your Name / AppleID] > iCloud > [Apps Using iCloud] - enable each native App/service as required

Yes, your iPads can be used independently and simultaneously.

Given that your iPads are of different age, model and CPU/hardware specifications, it may not be possible to install identical Apps or App versions. Having different iPads with different combinations of Apps for different purposes is not unusual.

As for iCloud (or iTunes) backups, these will also be independent.

What I suspect you're seeking is a way to mirror the app contents and app layouts and settings across both.

There's no means to mirror iPad contents, absent using a tool such as Apple Configurator 2, or some other MDM package. Doing your own small-scale deployment. This approach is probably more work than it's worth, and any change made to your iPad contents has to start with the MDM package and be reloaded into both iPads.

The brute-force approach here is to back up one, and then restore the backup to the other—basically, the same sequence as would happen with a migration from an old iPad to a new iPad. But that'll then start to drift as the two iPads are used, and you'll eventually need to keep one iPad as a "primary" as any changes to the "secondary" will be overwritten.

Schools do something similar, basically mass-reloading each iPad in a classroom with the same contents. But that requires setting up a fair chunk of management tooling, too.

In short, no good way to mirror these.

Being the same model, your choice of Apps will be identical. Otherwise, the same holds true.