Sorry - but to the best of my knowledge you're not going to accomplish that in the Resource Sheet. While Project may look like Excel, it is actually a SQL database in the back end - so adding columns to things like the Resource Sheet are limited by the fields in the database.

If you are using Project 2010, you can print a list of resource's non-working time through a report. Create a new resource report and on the Details tab, select Calendar. That will show each resource and their non-working time exceptions.

The resource usage view also shows shading for each resource during his/her non-working time exception.

By the way, there is no direct method in Project for scheduling a task when both resources are available. It can be accomplished through Task calendars - but it is a work around.