Sorry for being a contradictory view point but I would personally take a different approach. I would spend the money and buy a true battery monitor like the Victron BVM-600 (LINK)
. This monitor comes standard with a 500 amp shunt for $194.65. The Xantrex Link Lite is also another option.
By the time you're done buying battery cables, DPDT or DPST, switches or high amp battery switches, and building a by pass for a 100 amp shunt, you could have yourself a beautiful new battery monitor that will give you lots more information and for not a lot more money than a by-pass.
Out with the old and in with the new would be my suggestion.
I've yet to meet any boater, though I'm sure they are out there, who's installed a battery monitor and properly calibrated it that regretted it.