Here is a link to a timer that will do up to 9999 hours. 30 days equals 720 hours.

http://www.crouzet-usa.com/catalog/syrelec/812815.pdf

The alternating relay just needs a trigger signal from the timer to change between one set of output contacts to the other. Some of the alternating relays will allow you to lock one pump out to overide the alternating process.

I would suggest hour meters to ensure that the run times are equal.

Larry C