Dawson and Pacey both want to have Joey in a romantic, exclusive relationship. They can't share. Sharing sounds horrible to them.
And in general, people don't see how anything but exclusivity could work. They want the girl all the time, so they'd be missing out if they only got her part of the time.
But here's the thing. In the show, for the last episode they skip forward in time 5 years, and neither is with Joey (yet). But both Dawson and Pacey are pretty much OK with that. They are both capable of getting over it, and having a happy life with no Joey.
So when you see it from that perspective, feeling that only exclusivity could work is silly. They can be happy with nothing, so certainly they don't need everything. Now compare with if Joey spent one day a week with Dawson, and one day a week with Pacey. In those 5 years, she'd have had 250 days with each of them, instead of zero. That's a lot of days. And as their lives went on, it'd be a huge improvement over having nothing.
The fact they can be happy with no Joey, proves they can also be happy with some Joey. They shouldn't think of it as "not enough", they should think of it as something positively good that they can have without anyone being hurt.
Having someone *all* the time is crazy, anyway. Everyone should have his/her own life. Everyone is different, so no two people have exactly the same interests.