Hi,
It is just impossible that two events occur "at the same time", unless you deem the event takes the time that the signal takes to go from 1->0 (or viceversa). Even in this case, one can discuss if "the same time" means things happening perfectly sincronous (in which case it is impossible again).
Perhaps what you want is to know what happens if the second event happens while the processor is executing the code for the first event. Guidelines:
1.- Make the interrupt service routines as short as possible, so that if a second event happens while they are being executed errors would be minimized.
2.- The processor will attend the second routine on leaving the first one, there wouldn't be missing events if you follow "1".
(I don't know if there's the possibility to store the time the interrupt happens while servicing another ISR. Any case, for normal applications -yours is normal- the error would be minimum).
Best regards.