The level of each player are not important if you are on same server and have the same password and use a blue Eyed stone. I tried this with a friend last night and got pwct:
This Demon's Souls remake adds a feature called password matchmaking which makes it much easier to group up with a friend. If you go the in-game Options menu, navigate to Settings, and go to the Network screen, you can turn on a password requirement.
Once you turn on the password requirement, you'll only see the summon signs for people who entered the same password as you, and any summon signs you've placed can only be seen by other players who have entered that same password. This is a fantastic way to pair up with a friend as it bypasses the possibility of having a stranger summon someone into their game instead.
Using this password system also bypasses the level requirements for seeing a summon sign. You won't have to be within a similar soul level to see the summon signs of someone with the same password as you, so there's no need to worry about accidentally progressing too far past your co-op mates. However, do note that if one player is a much higher level, they'll receive a debuff to try and keep things better balanced