You cannot create a second share of the same folder. You must put all of the users in the configuration of one share per folder. Here is some code from my Samba template file that has parts that you could use to accomplish your goal.
# A publicly accessible directory, but read only, except for people in
# the "staff" group
; comment = Public Stuff
; path = /home/samba/public
; public = yes
; writable = no
; write list = @staff
# Audited directory through experimental VFS audit.so module:
# Uncomment next line.
# vfs object = /usr/lib/samba/vfs/audit.so
Instead of "staff" you could have a group that has John and Alice as members. If you think about it a little bit and experiment a little bit you should soon have a samba.conf section that will create the share with the characteristics that you want.