1966 dual master cylinder

buzzkill had a later model duel master cylinder conversion on his car same as you are doing, if the mastercylinder fails duel reservoir isn't going to help, but if a break line or wheel cylinder fails a duel reservoir will keep you from losing all your brake fluid and keep 1/2 of your brake system functioning so you can slow down or stop

also a good idea to make sure the EMERGENCY BRAKES are in good working order, in case of an emergency
the most common problem with the emergency brakes are old stuck rusty cables so it is a VERY good idea to replace them if they are old, new ones are about $15 each side and you can find them everywhere (it is a good idea to take your old ones with you to make sure they give you the correct ones)