There are lots of ways to do this, and some experimentation will be necessary to find the ways that work best for you.

An obvious strategy to start with is to have one person playing the NPCs and monsters, and the other doing the game mechanics. I've done this myself, and seen others doing it, both helping a GM who's very talented at plots and acting, but a bit vague on game system mechanics. In both cases, the game mechanics assistant was playing a PC, and had no privileged information about the world or the plot, but your situation is a bit different.

You might well trade the two GM roles according to which NPCs are going to be involved, since it will be easier to play them consistently if the same GM always plays a particular PC. This will allow you to have NPCs arguing with each other convincingly, which is rather hard for a single GM.

For example, Alice and Bob are your two co-GMs, and Charlie, Dave and Eva are the players.

Alice might be the usual mechanics GM, and Bob the role-playing GM, but Alice would have some NPCs that were always hers, and Bob might take over mechanics duties when they were centre stage.

If the party split in two, one GM could handle each part, which does make many kinds of tactics for the players easier to run.

In scenes that were just large fights, Alice and Bob could take separate groups of opponents. They would have slightly worse coordination than is commonly the case with a single GM, but that would actually be realistic: coordination in fights is hard.

There are lots of ways to do this, and some experimentation will be necessary to find the ways that work best for you.

An obvious strategy to start with is to have one person playing the NPCs and monsters, and the other doing the game mechanics. I've done this myself, and seen others doing it, both helping a GM who's very talented at plots and acting, but a bit vague on game system mechanics. In both cases, the game mechanics assistant was playing a PC, and had no privileged information about the world or the plot, but your situation is a bit different.

You might well trade the two GM roles according to which NPCs are going to be involved, since it will be easier to play them consistently if the same GM always plays a particular NPC. This will allow you to have NPCs arguing with each other convincingly, which is rather hard for a single GM.

For example, Alice and Bob are your two co-GMs, and Charlie, Dave and Eva are the players.

Alice might be the usual mechanics GM, and Bob the role-playing GM, but Alice would have some NPCs that were always hers, and Bob might take over mechanics duties when they were centre stage.

If the party split in two, one GM could handle each part, which does make many kinds of tactics for the players easier to run.

In scenes that were just large fights, Alice and Bob could take separate groups of opponents. They would have slightly worse coordination than is commonly the case with a single GM, but that would actually be realistic: coordination in fights is hard.