The salt is usually quite visible to a hacker. It doesn't protect you by being unknown, it protects you by making your combination (salt + password) unique.
Without a salt, you have two risks: If you use a password with low entropy, then someone else might use exactly the same password, and a hacker can find there are two users with the same password. That makes it likely that both users are using a weak password, so it's a good target for password cracking. With everyone using different salts, a hacker cannot identify weak passwords.
And you can buy "rainbow tables", that is a database of password hashes. Without salt, your password which has intermediate strength might be found in a rainbow table with very little effort. With everyone using a different salt, that is not the case.
So which salt you are using isn't that important. Even if a hacker learns your salt, it's not that important. And if you have two different hashing algorithms, so the same salt combined with the same password has a different hash, it still doesn't help a hacker.
That said, the more sand or salt you can spray in a hackers face, the better. So if using different salts is no problem, use different salts.