I'd advise chilling your beer, putting 12 psi on one keg and shaking it a ton. Then doing the same to the other keg, and leave the gas on it for the night. Do it every day for a week leaving the gas on alternating kegs, and I guarantee they'll both be carbed.
You can't do 12 psi and then take the gas off, you won't get enough in solution. Leaving it hooked up is the way to go, but 3-4 days or even a week of constantly being connected is not going to carb it well enough either. Over pressurizing and shaking is likely to get you two overcarbonated kegs.
I recently carbed a keg in less than 36 hours by chilling and shaking at serving pressure.