To be clear, you can walk anywhere you want, on any Camino route, and in any order you choose. HOWEVER,
one can ONLY qualify for the Compostela if one has walked the FINAL 100 km on any acknowledged route into Santiago.
So, walking from St. Jean Pied de Port to Sarria qualifies you for nothing. But walking from Sarria to Santiago qualifies you for the Compostela. it is all about actually arriving at the religious focus of the entire pilgrimage...the relics of the Apostle Saint James (Santiago).
Practically speaking, on the...
- Camino Frances, you need to walk at least from Sarria to Santiago (118 km).
- Camino Portuguese, you need to walk at least from Tui to Santiago (107 km).
- Camino Primitivo, you must walk from the town BEFORE Lugo, Lugo is 99 km from Santiago.
- Camino Invierno, you must walk at least from Monforte de Lemos
- Other Caminos have similar minimal distance points...
Also, you do not need to get your Compostela immediately on arrival. You could finish all of your walking then queue once. If you are going to seek two Compostelas for doing two routes, each consisting of at least 100 km,
BE SURE YOU HAVE TWO DISTINCT CREDENCIALES.
Coming the first thing the morning after arrival is a good way to avoid the long lines. The pilgrim office opens at 0800, seven days a week.
Hope this helps to clarify.