The traditional 80/20 approach to DHCP server availaibility works well enough unless it takes longer than a few hours to get a failed DHCP server up and working again. Or unless you've configured leases shorter than three days because your network is a classroom, a conference facility, or has a lot of mobile users who connect to it remotely. Because of considerations like these, many network administrators prefer a more conservative 50/50 rule, which actually works even better in a single subnet environment anyway, provided both servers have enough addresses scoped to cover the needs of all the clients on the network. Then if one server goes down, the other one can take up the slack for as long as it takes to get the first one up and running again. The typical way of implementing this is to use exclusions, for example:
Server 1 Scope 10.0.0.1 to 10.0.0.250 Exclusion 10.0.0.126 to 10.0.0.250
Scope 10.0.0.1 to 10.0.0.250
Exclusion 10.0.0.1 to 10.0.0.125
But you can also simply configure the scopes separately as:
Server 1 scope 10.0.0.1 to 10.0.0.125
Server 2 scope 10.0.0.126 to 10.0.0.250
Just don't let the scopes overlap when you configure them!