Can teams use a sub if both teams agree that it's ok?

No. 

When one team asks the other team to ok a sub, this has the potential to blow up in our faces. 

Usually, the team being asked is put into an awkward position and feels pressure to just say yes, because otherwise they feel like they're not being sportsmanlike. 

Also, often a team will allow a sub, realize that the sub is better than they thought, then either ask to disallow the sub, or request the forfeit. 

Last, it gives the opposing team an out if they end up upset when a call or the game in general doesn't go their way, as they get to claim that the other team played with an illegal roster. 

It's best to just not allow it in the first place to avoid problems later.