In my experience, having co-GMs in my roleplay has been invaluably helpful.

These co-GMs weren't picked so much as helpers, but are instead collaborators in the RP itself. We had RP'd together a lot before, so we knew and trusted each other. We had RP'd in the previous instalments of the series, so we all knew what was going on. From the advice above, this matches up with good choices for co-GMs.

If you have a co-GM, it is important to be in constant communication with them. Have a dedicated Conversation thread. All the GMs need to be in on the plot and what twists you have involved for everyone, or else they can't be helpful. If you are not in agreement, you will have a hard time moving forwards, so be sure to settle any outstanding issues.

Co-GMs help in many ways. They share the responsibility of the GM, meaning less demands on you. If something pops up in real life and you can't post, or can't post regularly, then they can carry the RP along without you (another important reason for them to be 'in' on the plot and direction). If your timezone is out of sync with the rest of your RPers (a common problem for me), then your co-GM may be in a better position to make immediate responses if need be. They can answer questions for you. Co-GMs not only fulfil administrative roles, but also roleplaying roles too. If you have characters/groups which are important plot elements (and not controlled by normal players), then assigning some of them to each co-GM (and yourself) will grant each character/group a unique voice and make interactions less limited by your personal writing skill. If you need help deciding on how to move the plot forwards, having a chat with your fellow GMs can brainstorm good ideas and even develop ideas you did even not think of on your own. Co-GMs would also prevent you from becoming a dictator that ruins everything for the players. You keep each other in check, and ensure that everything done is good.

There is the matter of how many players you have. I would suggest that it is a bad idea to have more GMs than players. Because you plan with your GMs what you intend to do with the story, having more GMs than players can change your RP from feeling like a roleplay to something more like a collaborative novel. The whole point of GMing is to develop a story with players and characters who respond genuinely and often in unexpected ways to the events you throw at them. RPing is very much an organic process.