If Masbooq misses the tashahud whilst completing Maghrib salah
In the name of Allah, the most Beneficent, the most Merciful.
If a person finds one rakaat of the Maghrib Salah with the Imam and misses two rakaats then the masbooq should complete the remaining two rakaats by performing qaidah and tashahud in between. If someone omits it then sajdah sahwe will be necessary. (Fatawa Darul Uloom Deoband p.397 v.3)
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham.