I wrote an article some time back about getting the best out of Adwords. There was a section in it about split testing. Here it is!
Always, always have more than one advert running for an adgroup. This allows you to see which advert is getting the best clicks through rate. You can then change the lower performing adverts and try to beat your best one. You will soon find that you have some very effective adverts.
However to test properly, you need to set the adverts to rotate evenly. This is set in the campaign settings. In the ad serving option under the advanced settings, make sure you select the radio button for Rotate: Show ads more evenly
Just to clarify, with the split testing, if you normally spend for e.g. £50 per month on 1 advert, are you suggesting that you should allow a £50 budget for the second ad (total of £100 per month) or allocate £25 to each ad?