Thanks for all the replies. Might as well get this done right so this is the plan..
Will eventually need access to the sender units so going to drill out a 4" access port for each unit... Since it's under carpet will re-cover the access ports with these under the carpet.
Will be wiring the gauge at the helm and the gauge at flybridge in parallel with MOMENTARY DPDT so I can use the wires to the sender from the helm... i.e.. wire from the flybridge to the helm switch then from the helm switch to the senders.
The momentary switches will insure that both are not being used at the same time, which will give false readings. The double pole will allow starboard or port tank to be accessed individually.
This is not going to happen immediately, but will when "I get around to it"...