Can a transaction to an air-gapped device be signed from another online device?

I want to send nano to a cold wallet and sign the transactions from another device. Can it be done?

Yes, there are methods for doing that but they currently lack an easy to use interface. You can have a look at the examples below but they are a bit outdated and the mobile feature was never released. This will be improved!

Example 1 (Online/Offline computer):
Example 2 (Two mobile phones):
Example 3 (Move funds from a Ledger Nano): Guide