I don't know if you'll actually have 20 people, but if you only have say, 14, make sure to have the diners with the cheapest food/drinks be the non-discounted ones. Meaning, if you have four kids ordering kids meals, don't count them as one of the ten discounted diners. Figure out if the extra people will eat enough/spend enough to justify the second card. Once you do that math you can decide if the second card will be worth it.
If you really have twenty or more (how fun!) and most or all will eat adult meals or like adults, you will probably do best to get the second membership. (I'm sure my comments are obvious to most, but thought I would mention it just in case.)