You mean in battle?
Sector 7 Pillar - Reno
Gongaga - Reno and Rude
Rocket Town - Rude
Underwater Gelnika - Reno and Rude (disk 2 only)
Midgar Subway - Reno, Rude and Elena
You also encounter them without battling several times. Without battle:
Sector 5 Church - Reno
Sector 7 Pillar - Tseng (after battle with Reno)
Elmyra's flashback - Tseng (in Aeris' house)
Shinra Building - Tseng and Rude
Mythril Mine - Tseng, Rude and Elena
Junon east side - Rude (in a bar playing cards)
Junon west side - Reno, Tseng and Elena (in a bar)
Gongaga - Elena (before battle with Reno and Rude)
Wutai - Reno, Rude and Elena
Gold Saucer - Tseng
Temple of the Ancients - Tseng and Elena (Elena in pool vision only)
Icicle Inn - Elena
The Crater - Rude (onboard Highwind during shinra cut-scene)
Underwater Mako Reactor - Reno
Vincent's flashback in Lucrecia's Cave - Vincent Valentine (past)
Good enough?
.