When both are up it goes upwards when both down, downwards, what happens when one is up and one is down?

then the plane will begin to roll.

Normally, the elevators work in unison, either both up or both down, Only ''V'' tail airplanes have elevators that can go opposite one another and they are called ''ruddervators.''
The reason is that the elevators also serve as rudders and by having them move differentially will control yaw.

Unless the aircraft has some type of computer-enhanced flight control system such as modern jet fighters, there is a broken cable/linkage to the elevator and the plane will not fly much longer!

think lawn dart.

you use the trim tab and power to control the up/down of the nose, land and have the mx guys check the rigging of the elevator.

well, unless if it is a computer assisted aircraft meant to do that, it means that the elevator rigging is broken or damaged. u would have hell of a problem.

certain aircraft, in particular military ones, have elevators that can move in opposite directions.

when one is up & one is down, on an aircraft meant to do that, the aircraft will yaw a little & corkscrew or roll to the left or to the right depending on elevator position (down & up =right roll & up & down = left roll)

when one is up & one is down on an aircraft not designed to do this, vertical axis is compromised: the aircraft will have little or no control over its pitch. the aircraft may corkscrew or twist also.

This happened to Egypt Air flight 990. The general consensus is that the relief First Officer was attempting to crash the plane while the Captain was attempting to save it. The RFO pushed the stick all the way forward. The Captain brought his stick all the way back, believing that since he was able to bring his stick all the way back, the elevators were commanding full airplane nose up.

It was not so. In fact, they had split the elevators. The net result was no net airplane nose up command. There was probably a small amount of twisting force as well.

The elevators cant move out of symmetry. Therefore elevators only control pitch. The elevators are mechanically or electrically connected to ensure symmetry.

If there was a massive failure of the elevator control system causing asymmetrical deployment lets hope the plane is already on the ground otherwise it will be soon.

It will roll in the direction of the up elevator.

PS: Some aircraft have 'elevons' where each elevator may either move in opposite direction, or both move in unison in the same direction, depending on the inputs being fed to the system.

If you were looking at a DC-9 or MD-80, they have split elevators that sometimes go in opposite directions. The reason for this is that they are not actually attached to the yoke. They have trim tabs that control which way they deflect, so in flight they would always be together. It looks strange on the ground, but works just fine as soon as you reach flying speed.

If you are not talking about an MD-80, the other are correct.

Very few aircraft r actually designed to have elevators that operate in different directions. As one person said the V-tail bonanza uses this to operate as a rudder however not to the degree u r suggesting. The MiG-23 uses this configuration exclusively for roll controll when the wings r swept. Also as was said by someone here, delta wing aircraft use their elevons as elevators & ailerons for roll control. MiG-29 is use this configuration in conjunction with ailerons to boost roll rate. Quite simply, if no other control input were to occur, the aircraft would roll in the direction of the elevator in the up position as was stated by another person here. Most r at least partly correct.