Currently, if you craft a 447 ilvl 1handed weapon (Agi / Str) then it will remove the crest cost for dungeon and raid 1handers to be upgraded to ilvl 441.
This means you can, as an enhance shaman for example, make a 447 agi 1hander, loot a 2nd 428 agi 1hander from M+ or raid, and then upgrade it to 441 for a total cost of 4 crests for 2 weapons.
If you are a caster however, and you make an intellect 1hander at 447, this will not affect the crest cost of your offhand. Meaning to do the same 447 + 441 weapon pairing would cost you 6 crests, 2 more than 2x 1handers.
I’m not 100% sure how staves and 2handers factor into this but my guess is that staves don’t reduce the cost of upgrading offhands.
This feels like an oversight one way or the other.