You are right that once his glucose levels are under control he will not be as hungry. However, it probably will not stop him from getting into Roxie's dry food. We sometimes call dry food kitty crack. Once they have it they will crave it if it is available.
The best thing to do is to have both cats eating the same low carb canned food. One method that works to help convert dry food eaters over to canned is to start by mixing some of the dry food with the canned and the topping it with a layer of dry food. Gradually add less dry food in the bowl until it is all canned. Another one is to add low carb treats, such as freeze dried chicken or fish on top of the canned food instead of the dry. Or try a combination of both.
What canned food are you feeding them? Pate flavors usually have less carbs than any food with gravy.
It sounds like you have not started insulin yet. Switching to a low carb diet can significantly lower glucose levels. We do recommend home testing. You will use either a human or pet glucose meter to test before every shot and before they eat. First this will prevent you from giving insulin when his glucose levels are too low. This reduces the chance of hypoglycemia, which is very dangerous. Second, this will let you know how the dose is working and if an increase may be needed. We can help you with this.
Do you know what insulin you will be using? Long acting insulins work better for cats than short acting ones. Good insulins are Lantus, Levemir, ProZinc and BCP PZI. The first two are also for humans so you will get this from your pharmacy. If either one of the first two are prescribed, ask for a prescription for the pens instead of the vial. You will be able to use all of the insulin in the pens. With the vial it will become ineffective long before you can use it all. ProZinc and PZI your vet can order for you. They do not come in pens.
Also can you remove the "GA" from your title for this post? GA means "gone ahead" and is used when a cat has passed away.