Certainly it is a good idea to have included in DPOA an alternate agent that can step in should the first agent not be up to the job for whatever reason. That can all be done in a single document.
It's possible to set it up in all different ways, but not all the options out there are smart ones. Having two agents at the same time can sometimes be a nightmare, for example. It really depends on who the trustees are and how well they get along. I've seen plenty of instances where there were more than one agent and all the agents disagreed about what to do, resulting in fights, bad blood between them, and sometimes a lot of money spent on legal fees sorting it all out.
Have an experienced elder law attorney help draft the durable powers of attorney to ensure they meet the requirements of state law and will work the way the principal (the one granting the DPOA to the agent) intends. That will also help avoid future conflicts over it.
It is generally a good idea for the principal to discuss his/her wishes and intentions with the agent(s) he selects and other family members as well to ensure what the principal wants done is what actually happens. If the agent is not clear on that, he or she may end up doing something the principal would not want. And the whole idea here is to honor the wishes of the principal, not what the agent wants.