I got 2/5 as well and i followed this method. Please let me know if this seems ok to you.
Prob ( both couples not sitting together ) =
1 - ((probability that exactly one couple sits together) + ( probability that both couples sit together))
Total number of ways of arranging 5 people without any restriction - 5! ( this is also the total number of possibilities)
Number of Ways in which 1 couple sit together but the others sit in any order - 4! * 2 ( If one couple has to sit together consider, consider the couple sitting together as one unit. so we have 4 units to arange in 4! ways, within their unit, the couple can sit in 2 ways, hence 4!*2)
Similarly number of ways in which both couples sit together = 3!* 4 ( multiplication by 4 because of 2 couples involved with 2 arrangements each)
So finally we get (5! - ((3!*4)+(4!*2)) ) / 5! = 2/5.