In GURPS Magic (all editions have had the same rule, but I'm playing 4th at present), mages pay a reduced fatigue cost to cast spells at high skill -- skill 15 reduces by 1, and skill 20 by 2.
As far as anything I've been able to find, that means a pair of wizards can recharge each other's spent fatigue as follows: Wizard A casts Lend Energy (skill 15) and lends Wizard B one or more points to replace B's spent fatigue. Wizard B then uses the same spell (also at skill 15) to replace A's spent fatigue. At each repetition, the caster pays one point less to cast the spell than it restores to the subject, with the effect that two mages can fully recharge each other (with the last recharge lending only a single point at no fatigue cost) in much less than a minute (even if they know Recover Energy, recovering from near-total fatigue would usually take 15-20 minutes).
This could be easier and even faster if Lend Energy could be cast on oneself, but I'm pretty sure that flavor of cheeses is too stinky and has been officially disallowed...