If you activate a lifeline twice it will appear as nested, so you must deactivate it before reactivating.
@startuml
participant T1
participant T2
participant Main
T1 -> Main ++ #red: start
deactivate Main
T2 -> Main ++ #yellow: start
Main --> T1 -- #red : done
Main --> T2 -- #yellow : done
@enduml
I don't know exactly what you wanted to achieve.