Can I alternate between two Octane types 91 and 95?

If you read the manual carefully, It says minimal octane rating is XX (being what ever number from 87 and up). I believe it probably says or the highest possible rating close to this number. If your car is a performance automobile, you will benefit from the 95 octane rating. But be careful, because an engine will get used to this fuel and it is hard to run at a lower rating, you will experience lower economy and knocking after prolong use of this fuel if you try to go back.