Heartgard chews are not designed to be broken in half, quarters, etc. The active ingredient (Ivermectin) is not dispersed evenly through out the chew. You can try Sentinel or Interceptor which comes in a 2 to 10 lb. dosage. Also, giving heartworm meds every 45 days is as sufficient as every 30 days.
A good read:
Billion Dollar Heartworm Scam