Thank you, guys. Actually none of these solutions meets the requirement of using only the two mentioned smartphones. I think generating randomly adressess at the offline phone
might do the trick.
If you insist on exporting the private key then understand that knowledge of the private key + your xpub gives an attacker complete access to your wallet. So it is not safe.
Actually it's not on my plans to disclose the xpub. Also I don't see how an attacker could have complete access to my wallet having knowledge of only one address's priv key. The other addressess's priv keys and the seed will be safe, right?
My first thought was that Electrum Android would have a feature that would solve this situation.
Thank you all once more.
If I had any smerit left, it'd be yours.
EDIT: Grammar and layout.