How (un)safe is this?
It is probably OK. Even though, in general case, you would want a different password for any different use, in your case the things you protecting are nearly the same. The wallet will hold all the "keys", and the keys along form your wallet.
Suppose you have a selection of deposit boxes in a bank. Now imagine that there are no guards in the bank and anyone can pop in and open any deposit box. Now, expand this idea, instead of a single bank you have as many deposit boxes as there are probably atoms in the universe.
A key is a pointer to one of those boxes. No passwords, no locks, just the location.
Because the wallet and keys are kind of the same stuff you can use the same passwords. But really, in a world where tools like KeyPass, can generate strong random noise, there is no excuse not to use them, and not to have two different passwords.
If an attacker retrieves these files, can the attacker take a cryptographic shortcut because both files use the same password?
An attacker would need to get access to either of the wallet or the keys to get your money. Given access to any, means he will be able to transfer the funds elsewhere. The good news, he will need to know your password. There are no successful attacks known, that can decrypt the data without the password.