No, we didn't really have any access to cheat codes (this was pre-anyone-I-knew-having-internet), but we definitely did two-player all the time; one at the mouse, the other at the keyboard. We had a hard time deciding who'd get the carrot ship.

On a related note: opentyrian - Google Code
Full game, open-sourced, updated to use SDL so it runs well on modern machines. There's a Nintendo DS port as well that I'll likely be playing a lot for the next few weeks.