No. You can't join both. It is not recommended to disjoin and rejoin. what you may do is creating the same logon id and password. check this page for more details,
workgroup networking faqs
How can I share files between workgroup computer and domain/workgroup computer
.... 2) change workgroup name to match win2000 domain and the laptop needs to ...
www.chicagotech.net/workgroupnet.htm
Bob Lin, MS-MVP, MCSE & CNE
How to Setup Windows, Network, VPN & Remote Access on
http://www.HowToNetworking.com
Networking, Internet, Routing, VPN Troubleshooting on
http://www.ChicagoTech.net
Can a PC be part of both a Workgroup and a domain at the same time? If not
and if I am currently part of a domain but want to join a worgroup
temporarily, can I rejoin the domain or will my account be permanently
disabled?
Thanks.