An approximation is more than enough. Actually i could skip the big lists and do it only with lists with less than 10 numbers, if that would help.
The real motivation for the problem (sorry for not explaining before) is that I need both subarrays to have a sum over a not-so-big threshold (easily achieved with 2 or 3 numbers per array). So if the big array is really big I can assume that the two small ones will pass the threshold.

Thanks for your help.