Like Aaron said, get yourself a GPS & calibrate your speedos. This is a normal thing on inboards. Usually only needs to be done once in a while.
There are 2 speedos because the pitot tubes on the stern can get clogged with sticks or weeds. A quick shot in reverse, from a stand still, will usually clear them out. Or Skidim.com has better quality pitot tubes that clear themselves out more easily.