Why are two photosystems necessary?

A single photosystem can't provide enough energy for the electrons it transfers its energy to.

they are necessary because these systems absorb energy from the photon and then conserve the energy in the transfer of electrons .. single photosystem cannot carry out the transfer because of absorbing low amount of energy ..

Atp is is our little "pocket" of energy. of course, if you have too little, then it means you have too little energy, and you'll pass out. the way to not have enough, is to have no energy sources (glucose, fats, proteins). you can't have too many atp molecules. cycles like the krebs cycle produce it, but if you have too much of any substrate (or too little for that matter), the cycle stops and atp is no longer made (it stops until substrate levels are such that the process can go on). in any case, this is unlikely since atp is constantly hydrolyzed and remade, even while you're asleep. you won't have too much. if you have zero, you are dead or dying.

The casefor proofs