Can only join if a specific player is online

I was thinking on making a server with this plugin:
https://www.spigotmc.org/resources/doublelife.102694/

The plugin will link player's lives and health together. I plan to keep the server on 24/7, and I do not want players to be able to join if the player they are linked to are offline.

So the idea is, the plugin would be linked to the DoubleLife plugin, and would be able to check which players are linked together. Then the players would only be allowed to enter the server if both tried to join within an amount of time.

I use Paper and Velocity, and have a lobby. The players first join the lobby, and then join the server.
That means that the check to see if both players are online trying to join should be done on the lobby.

I run a cracked server, so the plugin would have to detect the players via their username instead of their UUID.

I also ask for the plugin to be as customizable as possible.