Hi,
They don't get pregnant, but the female will have a boxed kitten when both her and the male show 2 hearts at the same time.
2 hearts = 100% love, 75% happiness or more, 25% energy or more and awake.
Partnering the cats will not ensure they breed, it only ensures the breed with each other. (Cats you own don't have to be partnered to breed as long as they find a mate you own with 2 hearts within range).
If your cats don't breed, it's probably because they don't show 2 hearts at the same time. Most of the time, it's due to unsync sleeping schedules, so synchronizing their sleeping patterns might help.
Hikari