DFP Ad to display 50% of the time on one single page & 50% on the rest of the website

We have an advert which we have setup in DFP small business which we want to display in a double MPU space on 1 landing page on our website for 50% of the time, then appear on every other page within the website the other 50% of the time. We have created the ad in DFP, we have added the double MPU holder on our website and the ad is appearing but it isn't a 50/50 split, it will appear on the landing page but if you refresh is will show a different google ad whereas we want it to be static while it is displaying our ad. When the ad isn't being displayed within the double MPU (it's 50/50 time isn't applicable) we can display either nothing or generic google ads but when the ad is meant to display it should be constant.