My dogs appear to be in a literal pissing contest -

[hedgehog]

Two males, both fixed, have taken to marking all over the front rooms of the downstairs. It's hardwood floor, but with gaps between the boards so I'm sure some odor remains. The younger dog has grown some over the past year; from 1/2 the size of the older dog to 3/4 the size. Is he trying to become the top dog?

[femmocrat]

1. Maybe.... were they neutered as puppies or as adults?

They could still have some territorial instincts. Can you keep them out of those rooms?

My adult female has to pee over everyone else's "spot" outdoors. She was neutered at 6 months. Some dogs just do stuff like that! Fortunately no one marks in the house.

[Demoiselle]

2. I have a new rescue dog in my house. He tried marking a few times indoors.

I was warned ahead of time by his Foster, since she knew he'd been outdoors a lot before he was rescued, so I watched him like a hawk at first and I let him know in no uncertain terms (loud, stern "NO NO NOS!" with cold human alpha female eye contact) when he did it and after about 4 goofs ...he doesn't do it any more.
It is a pain, but you might consider a watch-like-a-hawk campaign for, say, a weekend when you can spare the time, when you and other human allies never let either of them out of your sight.
My boy doesn't have another male to compete with either. He's become fast friends with our girl dog. She, thank goodness, has never had an impulse to mark.
Now, if I could just train my male cat to stop marking the dog beds..........

[Born_A_Truman]

3. Belly bands ...

http://cavex2productions.tripod.com/bellybands.htm

30% goes to the rescue of your choice ...

[TrogL]

4. Do NOT clean it up with ammonia - that just makes it worse