More Sense with this theory (spoilers)

I think they both put a hit on each other.

I say this because at the end, when SLJ was speaking to both of them at the dinner table, he said: 'You both make me sick, youre sitting here like im the bad guy.' Or something along those lines.

He wasn't taking to the wife only, so he is implying that they both have evil worse than him. At this point, Felton is looking real worried and cannot make eye contact, he looks left and right towards the ground, he is guilty of something and is afraid it will come out.

This also fits with the whole theme of the movie; what an average person would do when desperate: Murder. So both Felton and his wife put a hit on each other when things got to a breaking point, and both had other partners to continue going with after the insurance comes through.

The theory that they are both the same person is out the window. Who was it that murdered the cop and the two others at the restaurant when Felton was being interrogated? Not only will not fit with the plot but its too farfetched. It will also make his girlfriend an accomplice to haphazard murder.

The whistling at the end; it implies that they are both as evil as SLJ or worse, at least he wasnt killing family.

This also explains why he would continue living with her (he's just as bad a her so he can't really blame her - affair and hit). It also explains why her closing line is the same as one of his opening lines in the beginning: 'Everything will be alright.'