Mathematically speaking, it is not quite the case that husband & wife voting

for different parties cancel out. Such a vote might affect a race. If

there are three candidates, for example, getting A–99, B–100, and C–101

votes with husband & wife not voting; and if husband votes for A and wife

votes for B, you now have a tied race, instead of a C victory.

And of course, if husband and wife register in different states…