When meds are close, like 5-10 lbs., I don't split. When they're WAY out of line, like your "up to 22 lbs." - I split in half.
The vets don't want you do that. They tell you that there's no way you can be assured of the proper dose if the med isn't scored for splitting.
If they can't assure you that the med has been sufficiently mixed to allow you to split a pill, how can they assure you that the pill you have has the proper dose in it and isn't a "placebo" while another pill is double or triple dosed?
I find it a good way to extend the cost of the medication. I want my dogs to be protected, but the cost of meds can be outrageous.