Is there a way to optimize this solution for closing two circuits with one button on a scale?
The best way to solve this problem is to use a microcontroller that has interrupt/wake support on a group of its GPIO input pins. These would be programmed to generate the wakeup interrupt when any of the pins detects a state change. 

You connect this group of input pins to the MCU input side of the keypad matrix. Pullup resistors on these inputs keep all the input lines inactive. Before going into the low power state you program all the MCU outputs that are connected to the other side of the keypad matrix to a low level. When in the low power state any key press on the keypad will pull down one of the inputs to the MCU and initiate the wake event. 

The scenario I describe is just for the duration of the time going into the low power state and while in that state. After the MCU wakes back up you reconfigure the GPIOs to normal scanning operation. If you want one particular switch to perform the wakeup then scan for that one as soon as you wake up. If not seen then just immediately return to the low power state. Depending on how programmable your GPIO inputs are it may be necessary to poll for the switch release once you have begun the re-entry process for the low power state.