Buy a single or double kayak
If you always surf with someone else and the rapport is good, then yes, it’s worth considering buying a double kayak for two kayakers:
The double kayaks are more stable and fast for its greater length. But these types of boat are awkward to maneuver, having the weight of the kayakers closer to the ends. Since the wind affects them and the weight of the kayakers may vary, the rudder is usually a necessary accessory. It can also be more of a struggle having to load and transporting them because of their size.
Then you need to consider the double kayaks are harder to use for the individual and not handled easily by one person, so if you lack the companion … Solo paddling a double kayak is a demotivating experience -> Everything for what is done.
However, double kayaks are more stable and are often more comfortable and secure with a choppy sea kayak single.
So two single kayaks or one double kayak? Be sure to try both options in the case of a pair of paddlers. But also try to go out on your own in a tandem kayak and see how it handles. This should give you a good feeling on which way to go with your kayak choice.