My school used to ude that design... It's a more secure mechanism for master keying; two independent plugs each with their own pin stacks and possibly accepting completely different key blanks.
A gearing mechanism is used to transfer action from the upper plug down to in line with the lower, and the tailpieces of the two plugs are stacked and occupy the same soace tat the single tailpiece of a normal cylinder would. That allows installing this double-keyway cylinder into mortise locks originally designed for a mormal single cylinder, so the only additional cost is for these cylinders.
Good product for its time, Though rarely used except in the most complicated master-keyed sites.
These days most such sites have either gone to high-security interchangable-core, or past those to electronic, as less expensive yo maintain.