I'm trying to select two midpoints to set them with a vertical mate (so the middle of the lines stay together.) However, when I try to do so, I can't select the second middle point, it always unselects the first one despite whatever I try to do. I've tried holding shift or ctrl, but that changes nothing.

Is there a way to select two midpoints? Is there a better mate to use in this case that I'm not aware of?

Thanks!