The first comment fills in a key missing piece:
> The Russians had two intelligence organisations ... the KGB and the GRU. Both organisations knew that they should not reuse one-time pads.
> [Against clear orders] The printers decided to print four copies of each pad then send two each to the KGB and GRU. Neither the KGB nor the GRU reused the pads they received, except perhaps because of occasional operator error.
> ... Subtracting one message from the other cancelled out the unknown key to produce a synthetic message that was the difference between the two original messages. These could then be picked apart using a combination of statistics and predictable words.
> - Mark Lomas