Why would someone put 2 catalytic converters on a 1991 ford ranger 6 cyl. 3.0?

No. Single exhaust

Well, it didn't work! I've tried 4 times now and it still hasn't passed!

yes but maybe it did b4.get a diag on emission from smog ck station and post results and readings so we can adv

1st test: Failed HC(PPM) Max 124, Meas. 176. 2nd test (different station): Passed emmissions, Failed "Visible Smoke". 3rd test (free retest): Passed emmissions, Failed "Visible Smoke". 4th test (same station): Failed HC (PPM) Max 124, Meas. 125, failed Visual - 2 catalytic converters.

try repl with one 3 way catt. you have 2 two way catts right now