why would you even off hand the twashtar? all you gain out of it is some dex?
Because thf melee damage is still 50-60% of your overall damage so melee DPS is a big deal (unlike say, my drg who is about 20% melee, 5% jumps, 75% WS damage) and speed means more DPS and exponentially TP gain with those low delays compared to the slower alternatives (thokcha 190, corusc189, lolWhiteVWDagger 200+ etc)
For example, with 33% DW and ignoring other bonuses on the weapons, 99Man/99Twash has 9.7% more DoT (minor variance depending on Fstr. This is with 0) and 3.7% faster TP/time than Man/Corusc. 5% crit rate wont beat 9.7% more DoT let alone faster TP. The gap will actually be wider once you include the triple damage procs on mandau because it will skew the damage up based on the number of mandau hits which a slower offhand will literally slow down and make less frequent. Not a game changer but notable.
Thockcha stands a better chance in the DoT department since its essentially the same delay as coruscanti, but has lower base damage which is largely compensated for by the 11 str. Best case scenario 11str gives you 3Fstr on both hands up from 0. This puts Man/Twash with 6% Dot lead and 3.7% TP lead over Man/Thock. 26-27atk remaining to make up the gap of BOTH DoT and WS frequency. at 600atk (before thock) that is 4.5%+ damage (4.5% min, more if mob is higher level than you). But if were going to count the atk, then you have to consider the 10 acc from 20 dex on twash. 5% higher hit rate which is both DoT AND TP gain (and potential Ddex boost for crits).
They come pretty close with Twash winning in both atk capped situation (Trash mobs), Fstr capped situations (abyssea), and low acc situation sicne 5% hit rate from teh dex, and in the (very rare) situation that 20 dex makes a huge difference in crit rate. Thock wins if: acc is capped, fstr uncapped, atk uncapped(and pref vs high def mobs) and Ddex isnt in a favorable range for +20 to make a big difference. But even when all those factors are true, its not beating down twash by OMGBBQ amounts.
Twash will generally have the upperhand in most situations by a pretty fair margin. Thockcha will (occasionanly) beat out twash, but only slightly, and only sometimes. If i can only have 1 Man/Twash is the general winner. If looking at specific situations, carry both, but you will mostly use Twash anyway.