Dont count on your LFS taking in fish that are too big. In the long run, that'll bite you in the ass and you'll be stuck with 2 Triggers that will die from being in a tank that's too small.
They may say they will, now...but, not many "bigger" fish will be sold to someone. They'll end up rotting in your LFS's 20gal. holding/display tanks.
A 40gal. isnt even good enough for 2 juvenile Triggers.