Post.where(:comments.matches => {author: /asdf/}).
     where(:comments.matches => {body: /asdf/})
# => #<Mongoid::Criteria
#  selector: {:comments=>{"$elemMatch"=>{:body=>/asdf/}}},
#  options:  {},
#  class:    Post,
#  embedded: false>

Post.where(:comments.matches => {body: /asdf/}).
     where(:comments.matches => {author: /asdf/})
# => #<Mongoid::Criteria
#  selector: {:comments=>{"$elemMatch"=>{:author=>/asdf/}}},
#  options:  {},
#  class:    Post,
#  embedded: false>