You can't cast antimagic spell on the twins. The spell as you said, only works on your caster. What the posts are refering to is the use of a scroll. During the course of the game you have an opportunity to buy, or find, a scroll of protection from magic. This works as an antimagic shell. If you place the scroll in your inventory and then cast it at a twin, they will not be able to cast magic.
Hope that clears it up
I have one antimagic field scroll and that did not work either... but I did have a protection from Magic and used it earlier in the game... that is the scroll you are talking about, Right?
Yes. Antimagic scroll acts the same as antimagic spell. ie only on your caster. Protection from magic can be cast on anybody and is the scroll that you need.