Yes, the window widths must be twice as long.
The duration is defined by the factory as the interval in degrees (or seconds) from when the rotor cracks open .002", gradually exposes its full width to the housing slot, passes by and gets to .002" of closed on the other side.
Only the circumference (not the width, since the contact is on the outer rim) determines the duration, and this is the total of the rotor width + the housing width.
The maximum size is the smaller of the 2 slots, so you can have the same duration with a really wide housing slot and a .002" rotor slot, a really wide rotor slot and a .002" housing slot, etc.
The stock housing slot for all pumps 1937-71 is 11/32" (.34375") wide, or 81° since the rotor is exactly 1.00" diameter. The stock 45, UL, K, KH and 1957-71 XL slot is 3/8" (.375") wide, or 88°. The duration is 169°, only the opening and closing points vary between these engines.
The maximum opening is the (smaller) housing at .34375".
The 1/4-speed housing width is .270°, or 126°. The 1/4-speed rotor width is .214°, or 99°.
The maximum opening is the (smaller) rotor at .214", only 62% of the width (actually, the height is also greater, so the area comparo shouldn't be based entirely on the widths).