I'm not implying - I'm stating it
Virtually every boat I've seen throughout the Northeast (LIS, Buzzards Bay, Mass Bay) hanging on a mooring uses two pennants, so I'm following the norm here. I grew up boating inland in Louisiana and Texas where moorings are non-existant, and when in Connecticut my older boat was in a slip - so I've only really done 'serious' mooring the last 7 years, so I guess you can say I'm a relative noob in that respect. Slips are a rare commodity here in Marblehead and Salem Harbors north of Boston.
The one time we were on a guest mooring with a single pennant, I found out that the boat sails very aggressively, even with a stub of the mizzen up as an anchor riding sail - just exactly the opposite of your boat. In 25kts of wind, I was worried her 26 tons would make short work of the mooring's ground tackle.
I just found this
document from INAMAR, and it calls for primary and safety pennants, with the safety pennant being the longer of the two. Funny thing is, they state if was Marblehead, MA who came up with that recommendation - that's my home port and I can't remember seeing any mooring in Marblehead Harbor with pennants of different lengths! I guess it goes to show not everyone agrees to a single right method...
And the 18' feet I was thinking of was not to increase scope of the mooring's chain to the bottom, but to give better "scope" to keep it away from the dolphin striker. And for the snubber, I was thinking of placing a chain hook in the middle of 50' of 7/8" nylon and cleating off each end to create a bridle. That way I could vary the length as conditions dictate and to spread out chafe (in addition to chafing covers, btw).
Rick - I've seen both terms used roughly equally. In fact, I called them "pendants" until I was corrected by a crusty old salt!