I have a couple different phones...I've only used natrium on my android. Can I download the natrium app on my iOS , put in my android seed phrase and and install the same wallet? Can I access the same wallet from both?
Yes, seeds are usable in any wallet that supports the 64 character hex seed. Wallets that use a secret phrase may not always be compatible depending on the word set/format they use.
Just make sure your account is caught up before you try to send a transaction, so I would recommend not using both at the same time, but switching between them as needed should be fine.