All dry is really bad for diabetics and some kitties are even more sensitive to carbs than others. We have a couple on here that if they get so much as a mouthful of dry food they will bounce up into the 400s. So I'm suspecting that is what is happening here.
I'm by far not an expert, but for my two cents, if this was my cat this is what I would do. First switch at least the diabetic, if you don't think you can switch everyone, to a low carb/high protein wet canned food or even a raw diet. Start over on the dosing at 1u bid, hold for a week while testing like crazy to see how the dose is working. Just getting rid of the dry food may drop him/her quite a bit. Get a spread sheet up and attached to your signature like the one on mine. Instructions on how to do that is in the Tech section on this board. Plug in the numbers you have, it will help us help you and save us asking the same questions 100s of times. Then with this new data, we can help you adjust the dose as needed. Also go over to the Lantus support group here, and at the very least read all the stickies there, tons of info on how Lantus works, even pop in and say hi to the folks there, they all use your insulin so can give you very specific help, but they are pretty numbers driven so they will want a spread sheet that they can look at to help you.
My guy was 485 when Dxed, now a month later, with really not much more than getting him off dry food and a very short course of shots he is well in remission. That is why I'm suspecting there are a couple of things happening here, 1) the your kitty is carbs sensitive and the dry food is causing him to go high 2) He is getting too much insulin and constantly is going into rebound, where the body tries to protect itself from the overdose of insulin by releasing more glucose causing the irratic swings. Again fwiw I would switch the diet and go to canned food below 10% carbs and drop back to either 1u or even .5u for a few cycles to see if that helps. Really at this point what is there to lose?
Mel & Max