Usually when a person is having a Grand Mal siezure ( the whole body involved), when the muscles go into spasm the jaw clenches down too, making it difficult if not impossible to get the mouth open to deal with an obstructed airway at any rate. In addition there is a period of time during a full body siezures where breathing stops anyway due to the spasms of the diaphragm. If the siezure lasts more than a few minutes or if the person seizes repeatedly without regaining consciousness, you would need to attempt some form of ventilation (bag mask resuscitator, etc.)
which might push enough air around the obstruction to ventilate the patient at least until the obstruction could be dealt with. There used to be a device called the Oral Screw (no, I'm not kidding!) that was essentially a plastic screw used to pry the mouth open
to maintain the airway during a siezure. I suspect that more than a few patients broke teeth on it, as I haven't seen one for several years now -- hmmm. I have never been confronted with both of these problems at the same time, but it is possible...