If they both believe the same thing on one topic, then you can cancel that topic out and only look at the other policies

This isn't a math equation, it's a negotiation between human beings. You're saying that opposition to genocide is off the table because neither politician is offering it. But what happens if a sufficiently high number of voters say that genocide is off the table? In the short term, yes, it may mean a worse candidate wins. But if your goal is to stop genocide then it's necessary to create that impasse and maintain it until the other side caves. The fact is that we have something they apparently want, and there's no reason to hand over our votes if they open with the complete non-starter of supporting genocide.