I don't think there is, actually. The Divorce Act makes it clear that children come first. I was on the federal advisory committee to the deputy ministers of the Department of Justice back when the new child support legislation was introduced in 1997. The focus is on the children. The issue arises, in my opinion, because the Income Tax Act section I quoted, subsection 118(5.1), was introduced in 2007 to fix a problem with shared custody.
Because of the wording of the act.... Of course, the judges have to interpret the legislation as it is worded. It's tied to the wording of “a legal obligation to pay”. When the separation agreements are drafted, the drafter happens to say, “Well, instead of you writing each other a cheque, why not just write the net amount?” Just because the agreement says to pay just the net amount, all of a sudden there's only one obligation to pay the net amount. If that simple little wording is left out of the agreement.... That's the wording.