Quoted from dothedoo:

As you can see in the red box above, the solenoid expander switches between two solenoid busses. A coil on one buss shares the same ground line with a coil on the other buss. The solenoid expander determines which coil will fire (i.e. which of the two coils will have power) when a given driver transistor on the SDB is turned on.

The second diode prevents power from reaching the inactive buss. Looking at the diagram above, if the buss with the outhole coil is powered, as is shown by the solenoid expander relay, power will pass through the second diode on the outhole coil, through the coil and reach the ground line (A3J5-10). Since the outhole and saucer coils share the same ground line, power will also appear on the saucer coil. It will pass through the coil and if there wasn't a second diode to block the power it would reach the inactive solenoid buss, which causes all kinds of strange problems with the wrong coils firing at the wrong times.