I personally agree with opposite sexes if you only have two. Some dogs are ok with same sex pairings when one on one but why risk being one of the ones that don't, then you're potentially stuck with a lifetime of training and crating and rotating.
In the end there's no perfect recipe for matching dogs, we stack the odds in our favour as much as possible and hope for the best
Jasmine - Female Miniature Poodle - born Aug 15, 2010
Loker Delgado Von Stalworth - Male GSD - born Jan 26, 2012
Koda & Zazu - 6 year old male cats
Alex - Male Cocker Spaniel (rescue) - RIP Cuddlebug 2007-2010