Join the two PC's that need full access to each others folders and drives to a Homegroup. Do not add any other machines to this Homegroup.
Remove the Everyone share from the Security tab
of the special shared drive or else Everyone in the Workgroup will have access. You only want to share this special drive by using the Sharing Tab, set to Everyone, read/write as required.
You really should only share out the C:>Users folder rather than sharing the entire drive.
Both of the Homegroup machines will have full access to all shared folders and drives on all machines in the network but the machines outside the Homegroup will only have access to folders or drives which are shared to "Everyone".
This is the best and by far the easiest way to set this type of selective User sharing up.
Any questions let me know.