did you mean how as in 'where?' or how as in...well....'how?' the where was just covered, as to the how, well i'm a bit confused on why you don't know this, but ok, np ill try to answer this anyway.
there are 2 ways to do this. the first being a timed nuke. each of you get in range of <t> and at the same time (use the game clock or 1-2-3 go, w/e works for ya) then each cast same spell. *boom its dead*. you don't want to use spells that are too different in cast times, ie one uses tier 2 the other AM because the one using tier 2 will get sh1t kicked until the AM cast time is done. either both use the same spell, AM, a tier 3,or aga spell (if able to without pulling aggro but that is really rare), or what you can do is alternate who casts an AM while the other follows up with a tier 1 or 2. which ever is needed to finsih it off. just make sure the person not using AM doesn't cast till the AM lands or is just about to.
the 2nd way is to play ping-pong with the mob. this doesn't really work well till you have gravity and since i don't recommend you solo with /rdm till you have stoneskin, this might not be an option for you now. the idea is to stand far enough apart from your duo partner that the mob (with grav on) travels back and forth between you 2 as you alternate nukes. for example:
blm 1: /ma thunder iv <t>
blm 2: /ma gravity <t>
grav will hit first and mob will go towards 2nd blm. before it even hits him the nuke from blm1 will hit, turning the mob towards #1. soon as mob turns blm2 should start casting his nuke. before the mob hits blm1 (might take a blink shadow) blm2's nuke hits and since it now has hate from grav + nuke the mob will turn back to him slowly makings its way back.
this can be rinse and repeated as much as needed in order to kill it. now keep in mind this only really works when you have room for this (this works great in sea vs sharks) and typically at higher lvls. can this be uses at 53ish lvl? maybe but since 2 blms can 1 shot mobs its best to just stick to the 1st method. when the time comes that you can no longer 1 shot mobs, then its time to play blm ping-pong.
good luck and enjoy your blm solo or duo
bodhi the blm (blu now too)
the most cynical, sarcastic, bastard sylph has to offer