Ok, first off as a type 2 diabetic Im not sure why your taking a fast acting insulin twice a day! Humalog 75/25 as you probably already know is an insulin typically used by Type 1's that peaks in your body in 30 minutes and has a fairly strong response but is usually mostly gone in 4 hours. It falls off percipitously at 2 hours and at 4 hours has lost about 80% of its response from its peak! So that being said, IF you are getting an appropriate response from Byetta, using the humalog 75/25 could actually be dangerous in that it could cause you to go low very quickly! But Im not your doctor so I hope the two of you are really paying close attention to your 2 and 3 hour post prandial readings. I have heard of people using Lantus and Byetta as it gives you an extra insulin boost all day, but typically in 99% of the cases where Byetta is prescribed, it is prescribed either as a monotherapy or as an adjunct to Orals that have become inadequate. Because of the way Byetta bolsters beta cells and their reaction, its insulin producing effect, Im not sure why you would want to use insulin. This would only cause your Byetta to make your system sense that there was adequate insulin in your system and shut down the production of your own insulin (remember, this ability to adjust your own insulin production to the level of glucose/insulin present in your system is the chief asset of Byetta).
So the upshot of this is, have a conversation with your Endocrinologist and ask him why you are using insulin vs oral medications with Byetta, and if the use of a fast acting insulin may be counterproductive to what you are trying to achieve by taking Byetta! If you don't have an Endocrinologist, your regular internist may not really be up on what Byetta can do and how it is prescribed.
Let us know what you find out. Hope this helps.