You wouldn't be in breach of license agreement if you have three versions of VS installed and the two part time developers use the installs on those three computers, could prove to to be problematic with scheduling but legal... you could also have your part timers use The VS Express edition on their own machines, they wont have the functionality of the full version but it will provide them a dev tool without the license cost.
Larry Darrah - MS Architect Evangelist
Although I work for MS as an AE my opinions are my own and are not to be misconstrued as Microsoft's