Nick Wiant, Equipment Engineer at Midwest Express asked:

How far apart do buttons need to be that require the operator to push both before equipment will move? Is there a specific distance apart so that the operator can not push them with one hand?

SafetyMan says:

There is no specific distance required by OSHA. They state that the buttons should be far enough apart to prevent actuation by the operator using one hand and that actuation must require the use of both hands. Don't forget anti tie down and repeat must also be achieved in the two hand control design.