It has literally taken years to overcome this design obstacle, but I think I've done it! If you have not read my previous post, you should! The tl;dr is I've tried a couple of different ways to let the player control two characters, but they did not meet my standards for fun.
My latest attempt at Dual Character Control (DCC) appears to be successful. Simply put, the player controls Kidd and Fei just follows him. What kept me from implementing this before was the fear that the individuality of the characters would be lost if Fei's action were relegated to a single button. But putting them together did just the opposite!
The first thing that helps make this new attempt a success is eliminating frustration. Nothing breaks immersion quite like confusion, a lack of intuitiveness, and frustration achieving the desired outcome. The player is no longer required to learn a new suite of controls or radical mechanic since they control Kidd just like they have in the last two worlds; only now there's a happy-go-lucky Bomberman keeping up with him.
Bomb jumps are also coordinated. The player can continue to use manual bomb jumps as normal, but now there's a better way to do it. Further reinforcing the memory that she taught that, a separate button causes Kidd to place down a bomb, then the two jump on the bomb, and spring up really high as if bouncing on a trampoline. This "enhanced bomb jump" displays a technique that neither was capable of doing on their own while also being a really useful thing.
The Answered Questions of the Answer That Need Implementation
The final piece of the puzzle that will make everything work is providing opportunities for the player to use these actions. The screenshot below is of an early WIP. Kidd/Fei can only jump two blocks high. There are platforms that are accessible in a single jump, but many that a too high. This encourages players to regularly use bomb jumps. But bomb jumps are dangerous since the bomb explodes after springing on it. Done in confined areas, players are almost guaranteed to hurt themselves, discouraging usage of one of the primary ways I reinforce the relationship between the two characters. So there is plenty of open space to traverse and only a few confined spaces. Big, open levels are much different from the previous worlds' and incidentally help make that transition from the "tutorial" to the "real game" much more distinct.
There will also be plenty to kick, in the form of pushing green boxes, enemies, and a few destructible objects and switches that have since been implemented. There was a time when I was set on making specific enemies only susceptible bombs or kicks. Now I've settled on both being options, or at the very least one having a positive effect while the other seals the deal. One such enemy throws bones at Kidd/Fei. These bones can be blocked by bombs and bomb blasts kill the enemy. Since I've given the players so much to kick, they might experiment and kick the bone flying at them. Lo and behold, the bone will fly back at the enemy and kill it! Not only does this provide an avenue for different playstyles, but it allows me to design levels such that one method might be more advantageous to the other. This encourages and rewards experimentation and makes players feel smart, which is something I like when I play games.