Caution - a WAAS and a non-WAAS unit won't cross fill data. Also, a dual 430/W installation requires an STC for installation approval.
Best if you also upgrade your existing unit to WAAS as well ($3300 from Garmin in Southampton) and then you can get the necessary STC from Gama Aviation at Fairoaks. They have a separate STC which then approves the WAAS units for BRNAV, PRNAV and LPV.