According to the prescribing information sheet:
The recommended minimum dosage is 2.7 mg selamectin per pound (6 mg/kg) of body weight and 0.45 mg sarolaner per pound (1 mg/kg) of body weight.
Administer the entire contents of a single tube (or two tubes in combination for cats weighing over 22 pounds) of REVOLUTION PLUS topically in accordance with the following table.
|Body Weight (lbs)
||Tube Cap Color
||Tube Volume (mL)
|2.8 - 5.5
|5.6 - 11
|11.1 - 22*
*Cats over 22 lbs should be treated with the appropriate combination of tubes.
That confirms that the dosage for cat X is exactly half s much as for a cat weighting twice as much as cat X.
Example calculation: Cat A = 5 lb, Cat B = 10 lb
If both got a single dose, they would get one gold and one orange one.
Both cats combined (15 lb) should get a green dose split in two.
If both cats combined weight more than 20 lb, you need 2 doses anyways.
The problem is that you're not a robot and probably can't divide the dosage perfectly. I suggest treating the bigger cat first because you'll probably squeeze more of the product out of the tube at the first squeeze.
You should also apply the second dose as quickly as possible. Because of the alcohol content, the solution dries very quickly. If you wait half an hour to apply the second dose, the dilution of the solution changes and the second cat might get less of the active ingredients onto their skin.