I have a method that has worked well for me so far...I do a test on the EO/FO combination before putting it in a batch of soap using Q-tips and Zip lock bags.
For example, if I want to blend 1 part Lavender EO and one part Red Apple FO, I will dip one end of the Q-tip in the lavender oil and one end in the Red Apple FO. Then I put the Q-tip in a zip lock bag and close it up. A few hours later I will open it up and sniff. If I like it, I will use 50% Lavender and 50% Red Apple in my batch or adjust accordingly if I think it needs more or less of something. I always make sure I stay within the safe usage rates but the possibilities are endless; in fact I recently blended Patchouli with NG's Aloe and Cucumber (which doesn't sound so great, I know) and it was a lovely scent. Also, always make sure you write down what you blended and the amounts, this has happened to me where I had a blend that smelled great but I have no idea what or how much I put in there.
I've actually blended up to 3 combo EO's/FO's using this method and it has worked well, nothing has smelled like cat pee yet!
Good luck, let us know what you try!