For a slightly more "in continuity" look at the matter...
I'm guessing this FLASHBACK story in X-MEN MINUS 1 is the first time Charles and Erik have met each other since the flashback in UNCANNY X-MEN #161. That issue was pretty much it regarding the Xavier/Magneto friendship everyone gushed over. They knew each other in Israel for a while, they saved Gabrielle Haller from HYDRA, then Magneto took a load of Nazi gold and flew off.
( Arbeit macht frei (work makes free) )