Try these methods.
At the beginning of the meeting politely remind the members that the speaker will appreciate everyone's full attention and that during each topic covered there will be a period for questions and answers and that is the appropriate time for others to talk.
After covering a subject, remember to look up and around and ask for questions and comments. But then remember to keep the conversation on topic.
Sometimes all it takes is a glance in the direction of the talkers IF you can catch their eye. If not, don't be afraid to stop and say "excuse me Mary and Jane - is there something you need to add to the topic now?". It's a polite way to say "I heard you therefore it was disturbing" but at the same time give them an "out" if it was truely important to the topic.
Lastly, each speaker (if there is more than one) should speak loud enough to be heard by all. A strong voice is a good reminder to everyone else that someone else is indeed speaking at this moment. I personally have noticed a drastic reduction in "outside conversation" when a person with a strong speaking voice has the floor vs a person without. Call it "stage presence" or whatever ... it works.
It also helps if the speaker is standing unless talking to a group of less than 10.