That doesnt even make sense. Why would they be seperated?
Basically entire Wano Arc is Beginning of Final War, so we have:
Act 1 ---> Reunion
Act 2 ---> Wano Story
Act 3 ---> Onigashima Raid
Act 4 ---> Separation
Act 5 ---> Reunion Again
Once these 5 Acts are done, Final War Starts
However they won't be all Separated the same way, there will be different reasons for each, for example:
Luffy will be Captured by Fujitora & taken to Mary Geoise
Zoro will be Sent to Impel Down by Kuma
Sanji will infiltrate Vegapunk's Island/Lab or go back to Fishmen Island (Alongside Jinbe)
Rest of SHs will also Split by getting Captured + Going back to Zou + Going to Elbaf (The Place where they would Reunite) ... etc
The reason of this Separation is because of Multiple Events & Multiple Characters getting Introduced at same time
And Oda needs Straw Hats Split to be able to introduce everything well in preparation for Final Arc