Quoted from Eskaybee:Lyman's first solo game was TRON. Lonnie's first solo game was TF. (After the budget era when they were working as a team). Waison didn't enter the picture til xmen. Lonnie got re-involved for the final update which he needed to so that the game could be what it is today (a good fun game).
To be very technical there is a huge difference between "solo" and "lead", as not one of the games you mentioned was a solo effort according to IPDB:
Tron Legacy "Software by: Lyman F. Sheats Jr., Lonnie D. Ropp, Tom Uban"
Transformers "Software by: Lonnie D. Ropp, Waison Cheng"
X-Men "Software by: Waison Cheng, Mike Kyzivat, Lonnie D. Ropp"
Lyman obviously had several notable "solo" projects before he joined Stern, so I assume you mean "at Stern" for both guys. So obviously Waison did have some experience before X-Men, enough to get him lead on XM, but still had two others involved in the code. Any one of the three could pick it up to finish it off.