each dance/song costs extra mp for every dance/song you already have
there are different tactics how to "reduce" (its in "" because its not really reducing, but more like balancing MP between bards), because by going 1-3-1 for example, one bard will have 1 very cheap s/d and 1 very expensive while other bard will have 3 dances for middle price, thus both bards will end up with similar mp at the end. it depends by how many s/d each of bard is doing, for example in mage party, you pretty much want to go all songs first and then dances (or maybe fury somewhere at start so you prepare faster for pvp for example), but that way sws will have like 7 songs which are somehow normal price on average while BD will have 3 very expensive dances, however if it was done randomly or 1 by 1 changing, BD would end up with almost full mp while sws would be drained if he got like 2-3 expensive songs out.
what some ppl do is they either split s/d (done mostly in pve, coz in pvp you dont have time to do that) - songs and dances are 900 range, that means if you put sws on 1 side of room, in the middle your party and on other side of the room BD, party will be in reach of both songs and dances, however sws and BD will be out of each others range, that means they will get only their cost s/d (which is lowest possible) and its not like you need s/d on bards in pve anyway.
in pvp some parties are doing tactic to kick one of the bards before s/d, let the bard which stayed in pt sing or dance (depending which one stayed), after invite second bard and let him do his songs/dances = both bards spent lowest possible mp for their s/d, however this works only for duration of first s/d, because the bard who was in party whole time got the other guys s/d at the end, which in case of resinging/redancing takes too much time to wait for those to get out, but still cheaper than keeping both guys in party whole time.