Have you tried creating a 12 hour shift (10AM to 10 PM) with a break (or meal) between 2-6 PM?
Your agents who work the shift would have 2 choices, either to select the break/meal withdraw state (to remain adherent) or log off. If selecting log off, you would need to make sure the break/meal schedule state group within WFM config utility accounts for this (under the adherence rules tab)
As an addendum to this thread, I would just like to let everyone know that Split-Shift functionality will be greatly enhanced in WFM 8.0, which is slated to be released in June of 2010.