You can allways return to the right rep order to get both to exalted.
1. BSB till exalted (with BootyBay Bruisers / Faldir's Cove)
2. Goblin rep from -36.000 till 11999/12000 rep for BootyBay (grizzly hills - ship, instant respawns)
3. All Goblin rep from 11999/12000 till 999/1000 exalted in tanaris
If you read some of the postings from wowhead about "Insane in the Membrane", it seems like the easiest achievement ever, because all you have to do is "done in a couple hours".
I did most rep farming as HUMAN with GUILD BANNER and grinded as hard as possible. It took me over 40+ Hours for each rep.
Even Ravenhold from 11.999/12.000 till 999/1000 takes at least 30 Hours of ultra efficient LBRS speed lockpick runs + the homestoning/mailing time. I used a good mouseover lockpick macro and a junk-DELETE-macro.
People still complain about the new MoP Reputations and how much time you need to spend for them. Most of the Insane in the Membrane Reputations need more time investments than all MoP Daily Reputations together.
The time needed for this achievement is trully insane.