The relationship here starts basically during Memnoch the Devil, or at least that’s when Lestat first takes notice of it; with some jealousy. From what I can figure, Armand and David get along because they are both old men trapped in youthful bodies.
“…Armand shook his head, flashing a glance on David that implied they knew each other very well, maybe too well. I didn’t like it that they knew each other at all. David was my David, and Armand was my Armand.”
From what I can figure, Armand and David get along because they are both old men trapped in youthful bodies.
Their relationship continues when David interviews Armand and they together produce the 6th book The Vampire Armand. We see little of the relationship beyond this point as Armand goes off to his villa we presume and David goes off with Lestat, Louis and Merrick.