One doctype filled by multiple user

Frappe Framework Permissions
1

i have one doctype with ( 4 doc field foe example )
if there are two users to fill this doc
user A will fill docfield 1 and 2 and see field 3 and 4 as read only
User B will fill docfield 3 and 4 and need to see field 1 and 2 as read only

I tried to set this but save button not shown for user B

my setting was

user A has permission 0 with read and write for field 1 & 2 and read only on 3 & 4
User B has permission 1 with Read and Write for field 3&4 and read only on 1&2

when user B access the Doctype he can edit field 3 & 4 but cant save the Doc as save button not available

2

i solved this by
1-create docfield with permission 0 & both users have permission on it
2-fields for user A will get permission 1 with edit and create and permission 2 read only for user B
3-fields for user B will get permission 1 with edit and create and permission 2 read only for user A

Now both users can create and save