The ingredients are not evenly distributed within the pill/chew therefore you could cause a reaction rather than prevent one.
If a nurse administered a medication in the manner you suggest, she would be written up for a medication error.
“When splitting oral heartworm medication (especially soft chews), it is possible that you will not divide it evenly thereby resulting in one dog getting a smaller or larger dose. Additionally, oral heartworm medications have the active ingredients blended throughout and while theoretically the active ingredients are evenly distributed, there is no guarantee that there will be an equal concentration of the medication in each part. So, even if you’re able to split the chew exactly, one half could still have a higher or lower concentration of the active ingredients”
