Your good questions are contribution enough!
If all the leaves on the taller cane have died, you can be 90+% sure that it is dead. If the bark around the bottom of the cane feels loose and papery, then it is definitely dead. Remove it by slowly twisting the cane in place until it spins completely free. Then gently pull it up and out. Fill in the hole that reamains with the surrounding soil. Chances are you will find the soil down in the hole is quite wet and therein lies the problem.
It is more common for the shorter canes to die before the taller canes because the shorter ones don't get as much light. When the reverse happens it is usually because the taller cane was not as well rooted as the smaller one when they were potted together.
That said, the fact that one caen has died is a good indication that the other one may follow, albeit a bit later. Thus, it is important to figure out what went wrong.
Corn
plants do best in bright indirect light. They will survive low light, but only if the soil is kept very dry. In bright light, allow the top third of the soil to dry out before watering thoroughly. In low light, allow at least the top half of the soil to dry out before watering.
A final note, corn
plants have very small root systems and never need repotting. If you repotted yours (a very common mistake), then you can be sure that the excess soil you added has retained water for too long and caused the roots to rot.
