Keys that share a common address line or a common data line with
a Shift key can be combined with a simple passive diode circuit.

Common address line
-------------------

                  +--|<|------Data
Address---*key*---+
                  +--|<|------Data

eg:
A15: B N M SYM   SPACE
     * , .     SYM-SPACE


Common data line
----------------

                  +--|>|------Address
Data------*key*---+
                  +--|>|------Address


eg:
D0: SHIFT A Q  1    0   P ENTER SPACE
              EDIT DEL          BREAK

D1: Z  S  W  2 9 O L SYM
    : NOT <> @ ) ; =


No common lines
---------------
This needs additional active circuitry.