In Synergy, how can I configure a client monitor with a server monitor on either side?
This functionality won't be available in an intuitive way until per-monitor configuration with uneven edges hits in Synergy 2.1 (2.0 is still in beta at time of writing).
You should be able to fake this though, as long as you're okay with the PC screens 'wrapping' around.
For your specific use case, put PC screen 1 to the right of PC screen 2 in Windows display settings and then set up synergy to wrap around (PC is to both the left and right of Mac).