We don't know what the size of the pill is. If it's already a large pill because of the time-release coatings, it's better to have someone take two large pills than one giant-sized pill.

Also, making a 200mg pill might mean another study, to prove that it dissolves in the same place that the 100mg pill did. Why not keep it simple (and keep manufacturing costs down) by having one less pill to deal with?

Some people have trouble swallowing pills. I'd sure rather take two normal-sized pills instead of one huge one.