Do you and your DH always agree on discipline for the kids?

[Nabster]

And if not, who has the final say? What do you do if one has started a course of action and the other doesn't think it is right/fair/appropriate? How do you resolve that without underminding the other one?

No.

If one or the other disagrees, we leave it until we can discuss it minus kids, then the person who gave the punishment will change it if that is what we decide, or it will stay, and someone has to suck it up

Usually i have final say as i am with them more, so have to put up with crap punishments that DP gives without thinking of the consequences for me.