here is revised drawing I threw together.
The reason for using the #41 sized sprockets is they are easily mounted on a shaft with out doing any welding as with using a sprocket and a freewheel adapter. A large selection of sizes as well.
With this set up you will have a freewheel differential. Research my posts on the T bucket for welding freewheels using steel collars.
Hope this clarify's things. Your rowing / pedal set up is great, plan to use for a land rowing machine ride.