I own a v10 double and have used the v8 double. I would go with the v8 double if you are deciding between those two boats. The v10double is only faster with skilled and strong paddlers. The main issue is stability. I consider myself advanced - I paddle elite skis comfortably in all conditions. If I am with another advanced paddler we are amazingly fast in the v10 double, at least on the flat. With an intermediate paddler in the back it is a mixed bag. If they are lighter than me it goes ok. If they are heavier than me the boat is wobbly and much of the speed advantage of a double is lost. With a beginner in the back, It goes fine if that other paddler is a petite woman who is too light to throw off the balance much. But with a beginner in back who is my weight or heavier it is utterly horrible. The boat wobbles violently, and the only thing keeping it upright is my own core strength and balance. It is a slow, frustrating, joyless experience that leaves my shoulders and back sore for many days.
As far as speed, the v10 double is 0.5 - 0.7 mph faster than a single elite ski assuming there are two equivalent paddlers in it with no balance issues. The v8 double is the same speed or perhaps 0.1 - 0.2 mph faster than an elite ski, assuming equal and highly skilled paddlers in all craft.
Two beginners in the v10 will be swimming a lot. Two intermediates in the v10 might do ok if they have great balance, but will probably be slower and having less fun than they would be in a v8 double where they could lay down power.