Like Taz said, 40gal wide will definately be large enough. 55gal would be better. Length, and width are very important in this case, as the pikes will need to be able to get away from each other yet still be able to set up comfortable territories. Caves, driftwood, plastic or live plants are a MUST. There HAS to be hiding places in the tank, ways to break up the pikes line of sight so that they can get a break from each other with out being harassed to death. Pikes are generally 'out of sight out of mind' when it comes to each other.
Also, gettin a trio will work, but gettin 5 or more would definately increase your odds of success greatly!! Keepin the extra pike in the tank is a good idea as well as it will most likely strengthen the pairs bond together.
