What is meant by "same time"?
1. To the millisecond (impossible)
2a. Both must hit with the same score. Both getting 300 or both getting 50 result in a hit (100%). One hitting 300 and one hitting 100 gives a miss (0%). Needless to say, both players missing should result in a miss.
2b. Like 2a, but early 100/late 100 and early 50/late 50 are distinct. So if one gets an early 100 while the other gets a late 100, it is counted as a miss.
3. Calculate score for each individual note from the less accurate player (e.g. 300 and 100 -> 100).
None of these definitions really make the game a rhythm game anymore (mostly luck now, hoping the other player hit as bad as you). Definition 3 might hold up to some extent, but it's not exactly relevant to what's requested.