Only one of them can be the master, but since they're demons, this system has to benefit both parties.
When a demon becomes an apprentice to another demon, they can learn the master's skills.
If the master knows a spell, the apprentice will be able to learn it and make it their own.
Also, if the master is good with a certain weapon, the apprentice can get better with that weapon at an increased! (Aptitude)
Learn spells from your master!
Your Weapon Aptitude increases.
In return, the master's stats increase depending on the stats of the apprentice!
As the apprentice gets stronger, the master's stat bonuses increase.
Take in an apprentice and boost your stats!