Are you required to do a separate CLIN or can you just extend 2 months at a time up to 6 months? The base and option are for 6 month periods.
FAR 17.204 (f) & (g) speak to this pretty well. You could set up your option as a separate CLIN or you could (per FAR 17.204(g)) express the option extensions as additional time for performance (i.e. 2 months at a time up to 6 months). Either method of expressing the option is open to you. However, the option CLIN (or multiple CLINs) method is a clear way of showing the pricing for each 2 month option.