Basically, the gist is that manual Jump actions don't respect jump sustain. So if you tell an object to jump with an event (instead of the object receiving an actual a control press) then there will be no jump sustain, because basically there's no action that says "hold the key down for a while to sustain the jump." And since there were no official player 2 controls previous to .99, the only way to make a second player was to manually tell a second platform object to jump with an event.
Now, in .99 and above, there are proper second player controls that can be configured in the app properties, so you can set up real jump controls and have real jump sustain for a second player. So if you're lucky enough to be able to create and export your project with the unstable 0.99.2 build by the time your project is due then you should be golden.
But if you don't want to take the chance that you might not be able to finish an already risky class project then I suggest using 0.98.9 and just not having jump sustain.
And I guess that's all rather beside the point. Here's how you manually make a Player 2 with events:
Start of layout -> Platform2: Ignore Input (on the P2 object)
Key (move right key) Is Down -> Platform2: Set X Component of Motion to positive amount
Key (move left key) Is Down -> Platform2: Set X Component of Motion to negative amount
Key (jump key) Is Pressed -> Platform2: Jump
Those are just the basics, though. You will need to tweak it a bit to work properly.