Sorry guys but you can save your Visa entry for later.I did a trip out and came back in and the Immigration official asked me if I wanted to use my second entry or just take 28 days on arrival stamp.
I took the normal entry stamp and saved my Visa for later.
One word of caution. Be careful to check the small print on your Visa.When my friend and I drove to Sadao for a Visa turn around he was refused his second entry.
The use by date means that it must be done before the date on the Visa ticket otherwise you have forfeited it.