In MathWallet5 you can use import from other chain function to create wallets for different chains instead of creating multiple mnemonic words.
Here is the guide:
Let's use new Polygon wallet as example
Switch to Polygon and click 'Add Wallet'
Click 'Import a Wallet'
Click 'Import wallets from other chains'
Select the target chain
Select one or more wallets from the target chain, and import.
So that you can import any existing wallet on the other chain instead of creating and backup sth new.