Oh, I didn't mean that, if I were the host in this situation, I would comment on what someone ate or didn't eat, or that I would find it rude if they didn't eat the cake I served, only their own. I agree with all of you there. But you can happen to notice an untouched plate without monitoring your guests' eating, and you can get your feelings hurt without judging anyone as rude.
I was looking at it as if I were the guest, not the host. What would be the nicest thing to do?
I think that the host's feelings might be hurt if I ate a slice of the cake I brought and none of theirs, especially if they worked hard on it and were really proud of it. I wouldn't want to do that.
Plating desserts -- especially 2 slices of cake -- without asking the guests what they want just sets you up for this, though, doesn't it? I mean, that's a lot of dessert. Not to mention having a guest bring cake when you are also making a cake. If the host would just ask first, I would say, "Just a tiny slice of each, please," and then eat enough of the sponge cake that it wasn't obvious that I was polishing off the chocolate but leaving almost all of the sponge. If that meant that I got a little less chocolate cake into me than I would've preferred, that's not really a big sacrifice. It's worth it to be kind to my friend, in my opinion.
(And I am not a fan of sponge cake, either.)