Possible implementations:
1. Stacking Sugar & Spice bonus. The cumulative S&S of all involved players is used to calculate F&F reductions rather than individual S&S values as an argument to sqrt() (or whatever diminishing returns function is used). This option looks to be the easiest one to implement. This option gives more benefits to less advanced members, while still giving some extra bonus to most developed players.
2. Additional bile points for reaching certain cumulative feasting set values. This option looks nice at the first sight but can be easily overpowered, I do not like it very much.
3. Shared food group restore mechanics. E.g. When one player eats a food piece without getting a debuff all other players get a 5% chance to get 5% restore to the same food group. When one player gets debuff all other player get 5% chance to get 10% debuff. Eating larger amount of foods with more than 30% chance for a debuff will have negative effect on co-feasters. I like this option because it requires food discipline from all village members and involves responsibility for others.
Some math for option 3:
Currently it is not possible that two players access a feasting table at the same time. This has to be changed, instead adding any items to the table should be disabled as long as any of the players accessing the table is in gluttony mode.
In H&H there was also a fireplace mechanics binding the tables within fireplace range together to get even more bonuses. This can be replicated in Salem, however furniture used outdoors might be damaged during the feast. In the future a new building - feasting hall or canteen can be added instead of fireplace to allow big towns to feast indoors.
I am fully aware this mechanics will benefit raising alts. I do see how any mechanics benefiting other player could not be (ab)used to benefit alts.