I did it, too, when I used chemical flea control. Since I have Teenie, now, and it burned her the only time I used it on her, I don't use it any more. It helps a lot, with multiple pets. My vet will tell me the correct dosage for each, if they're dramatically different, and give me the syringes to use.
Amanda & Bevo 2015
Libby's Monkey N The Middle, CGC, RAE, BN, CD (Bean, Chinese Crested, 1/26/08)
Fluffy, Cairn Terrier, gotcha 6/10
Gaea's Pandemonium By Request (Chaos, Chinese Crested, 2/14/12)
Mischief N Miracles, RAE, CGC, #1 Parson Russell Terrier in AKC Rally, 2009 (Emma, 10/13/03-1/8/10)
Jewel, Italian Greyhound, 9/15/01-7/7/12