Is it possible with Postgresql to create a database which has 2 users which act like owners to the database?
I can create a group role and add both users to that group, and then make the group the owner of the database, but this requires both users to be have to manually set their role on every connection to make any tables they have created accessible to the other user. Is there any way to make the group be the default role for a user each time they log in or any other way to achieve the same thing?
No, each database can only have one owner. As stated previously you can have more than one superuser, or you can grant permissions specifically to group roles that are then inherited.
You might want to look at http://blog.hagander.net/archives/70-Faking-the-dbo-role.html, for a way to fake something similar to what you're asking for. It's not perfect, but it might be good enough for you. It should be able to solve the object-ownership problem at least.