Lets say the domain name is mydomain.com
You want to put the two workstations, assuming they are physically connected to the network, in the mydomain workgroup.
Share the folder on the clients pc. Create accounts on her machine for the two pcs users with the same names/passwords they use to logon to their pcs.
To start have the share with full control for everyone.
The two pcs should be able to access the share by \\pcname\sharename