You only get one match, unless as stated it is a program with a prelim year like radiology. The other difficult part is you can't apply for two specialties at the same hospital. I mean--you can but it's a bad idea because both programs will likely find out and both reject you. So your backup specialty ends up being not at your top or mid tier hospitals, because you applied there for your main interest.
I'm thinking of dual applying general surgery and integrated vascular. It's not as bad because there are fewer vascular programs than, say, ortho, but it will still foreclose opportunities for general surgery. Like if I want to apply to the integrated vascular program at Awesome Hospital X, I can't also apply for general surgery there. So if I don't match vascular, I'll end up matching and Not So Great Hospital Y, as a backup.
That's the sense I got, and the risk you take in dual applying. But hey, better to match into your backup than not at all.