Let's say I have two Ledger hardware wallets. Can I use the same seed phrase on both simultaneously and be able to work with my crypto on either whenever I want having access to the same accounts?
Yes, by using the same seed phrase (and derivation path), you can have multiple instances of the same wallet. A wallet does not 'contain your bitcoin', it just contains the keys that allow you to spend your bitcoin. So you can create multiple wallets that hold those same keys, and then use any of those wallets to perform wallet functions (like sending, receiving, etc).
Keep in mind: every instance of a wallet is also increased potential for your bitcoin to be stolen/etc. Make sure that you fully trust whatever hardware/software you are entering your seed phrase into, and also make sure you keep track of and handle all instances of your wallet(s) appropriately.