Cats can disappear for months on end and just turn up again!! My sister had one that was away for almost 3 years and then one day, she just came back!
We had 3 cats, 2 boys and 1 girl, I got another kitten and the younger of the boys left and never came back - that was 4 years ago, but my husband & I saw him several streets away about a year ago and think that he just went and lived with someone else. Mind you, when he first went missing I never slept for almost a month and every noise I heard, I was down the garden, even in the middle of the night, calling him!
I would love another kitten, but the female would not stand for another girl and I'm affraid to get another male and maybe the same thing would happen with one of the existing males.
I would say if you do go for a new kitten, get a male 'cos I think they bond better with whichever sex you already have, but wait awhile longer just to see if the other one comes back.