http://www.sheltermedicine.com/portal/is_parasite_control.shtml#external said:
Frequently Asked Questions about external parasite control
Is it okay to split large-dog size packages of Advantage, Frontline, Promeris or Revolution and use them on multiple smaller animals?
This is an attractive option for shelters because it is cost-saving, and may allow shelters to treat large numbers of animals with products that would otherwise be unaffordable. This approach is widely used, and appears to be safe and effective. However, using Advantage, Frontline or Promeris in this way is actually illegal, although it is acceptable to use this method with Revolution. This is because Advantage, Frontline and Promeris are considered pesticides rather than medications, and, as such, are regulated by the Environmental Protection Agency (EPA) rather than the Food & Drug Administration (FDA). Unlike the FDA, the EPA does not allow for use of products in any manner other than exactly as product label describes. The splitting of large sizes of Revolution to use on multiple smaller animals is off-label, but is acceptable as long as it is done under the direction or supervision of a veterinarian.