you can double the use of staves of charging. i'll explain with an example.
before equipping the ring of melian (which grants channeling) i had a staff of freedom with 4 charges and a staff of recharging with 3 charges. i equipped the ring and freedom had 8 charges and recharging had 6 charges. so far, this is all normal. i used the recharging on the freedom once and the freedom charges increase by 2 to 10 charges. after using all the recharging, the freedom had 20 charges. then i switched to a different ring and freedom had 10 charges.
unfortunately, i was not able to test what 2 staves of recharging would do.
before equipping the ring of melian (which grants channeling) i had a staff of freedom with 4 charges and a staff of recharging with 3 charges. i equipped the ring and freedom had 8 charges and recharging had 6 charges. so far, this is all normal. i used the recharging on the freedom once and the freedom charges increase by 2 to 10 charges. after using all the recharging, the freedom had 20 charges. then i switched to a different ring and freedom had 10 charges.
unfortunately, i was not able to test what 2 staves of recharging would do.