Quote from Crucis:
The reason for that answer was that it was the "new" requirement from GS, the OX closing agent at that time. Since then, OX has dropped GS and is now their own closing agent. Margin rules have now changed. They only charge for one side for an IC and similar trades. Check their website. The info is on their login page.
Cru
I spoke to someone at OX recently and they still charge margin on both sides of the diagonal. Something about long and short strikes being in different months. They only charge for one side on IC's.

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