Who wins a tie of epic fail?
If at all possible, neither of them. They should both fail. But if it's a test that has to go one way or the other:
- Whoever failed least
- Default goes to the defender/attacker
- Default goes to the players
Whatever you want, but once you pick one, keep it consistent.
(Crit) Glitches in SR4 are opportunities to let the DM add flavor to the game. They are toys for you to play with and throw interesting things at your players. It takes a non-trivial amount of spontaneous wit and creativity. If that's not really your thing you can simply generalize it to be the typical examples of "you fall down" or "your gun jams". It's important not to over do it though. In a low-powered game where the dicepools are small and critical glitches are common, feel free to "tone down" the repercussions. And remember that when you simply glitch, you still succeed at whatever it is you were trying to do, but with some added annoyance.
When both players critically glitch in an opposed test, then their actions work against them in some way and they fail at what they were attempting. If there is a directly opposed test of say, opposed strength checks where one guy is trying to get through the door and the other is trying to keep the door closed, one possible result of them both critically failing would be to have to door break and the whole thing falls over pinning the opener (but letting his friends through). The guy isn't through the door and the door is now open. But if it's the sort of test where it has to go one way, like a sneak/perception check, then have a system for resolving ties and then throw hilarity at them like Leezards story above.