I have searched for an explanation of this, but could not find one.  I understand why it is a good idea the have three CDUs and, obviously, an MSU for each CDU.  What I don't understand is why there is a need for two ADEU Cardreaders.  The plans/cards in cardreader 1 are identical to the ones in 2.  If I have a plan that requires multiple cards I can simply find Card 2/3/etc in Cardreader 1 and load from there.

I understand a need for 2 in real life, but in FS things don't break unless we want them to usually, so if Cardreader 1 worked for Card1  it will work for 2/3/4/etc.

Randy

Edit:  Please disregard, I realized when loading card 1 of 2 for a KSFO to KSAT flight that Cardreader 1 will only load cards in CDU1 and to get cards into CDU2 you must have cardreader 2 installed...unless, pressing "Remote" on all three CDUs before one installs Card 1 in Cardreader1 for CDU1 installs the waypoints in CDU2 and 3 as manual entry does???

The reason behind the double ADEU is fault tolerance.
In reality, and in the gauge, ADEU1 is hard wired to INS 1 and ADEU 2 is hard wired to INS 2. In case of INS 1 or 2 failure, you would still have one ADEU available.

Marco