Short Answer: Only if you want to!
Long answer: It totally depends on your relationship with your sibling. Remember that your senior portraits are a guide post in your transition to adulthood - how much is being a twin a part of your identity?
You might have been like me - I didn't really stand within arm's reach of my sister without some sort of verbal altercation taking place until after I moved out of the house (she's younger, but only by 14 months). Probably pass on sharing a senior photo session with your twin.
You might be like my best friends in high school - you don't talk about it outright, but your twin is pretty much everywhere you are, which can be good or bad, depending on the day. A shared senior session may or may not be a good fit for you, then - maybe you would be better finding a mini session to celebrate your twinness?
You might totally be each other's missing piece, though. If your twin is your ride or die in a way that a best friend is, with the added perk of living in your house with you, sharing a twins senior photo session would probably be a lot of fun.
I think it all depends on your dynamic. That said - this is probably the last year you share this type of relationship with your twin. Let your feelings about that help you decide.
(Their mom told me this will never happen again in recorded history)