The answer, of course, is, yes you can, but I will try to convince you not to! Obviously it is more convenient to mix your fast-acting insulin (like Regular, or lispro [Humalog], or aspart [Novolog], or glulisine [Apidra]) in the same syringe as your long-acting insulin (like NPH, or glargine [Lantus], or detemir [Levemir]) and give it as one shot instead of two separate shots. In fact some insulin preparations already come mixed together. There is an insulin called 70/30 that is 70 percent NPH insulin mixed up with 30 percent Regular insulin. We are supposed to believe that the 30% of fast-acting Regular insulin will be absorbed first, giving us a peak of insulin in the first 2-3 hours after taking the shot and that the 70% of longer-acting NPH insulin will kick in later giving us a second peak about 6-10 hours after we take the shot. The problem is that this is NOT what happens at all. After all, what is NPH insulin? It is Regular human insulin with "junk" added to the bottle to slow it down. the "junk" is a fish protein called protamine and a suspension of zinc molecules. When you draw up more Regular insulin into the same syringe as the NPH then this Regular insulin also gets "slowed down" by the cloudy mixture of protamine and zinc. Instead of getting an early peak from the Regular and a second slower peak from the NPH you get one single peak that happens half way in between when the Regular peak and the NPH peak would be expected. Now this works fine for some people but if you really want to get a quick peak to cover the carbohydrate you are eating in a meal and then get a slower, flatter second peak from the NPH insulin it works much better to give each insulin as a separate shot.
A recent study looked at this with two of the popular insulin analogs. Eleven people with type 1 diabetes who were taking the fast-acting insulin lispro (Humalog) and the long-acting insulin glargine (Lantus) were asked to take them as separate shots on one day and the same doses mixed together in the same syringe on a second day (This study was published recently in the journal, Diabetes Care, 33:1009-1012, 2010 by Cengiz, E, Tamborlane, WV, and colleagues). What they found was that the rapid peak from the lispro was lost entirely when it was mixed with the glargine insulin. This meant that the blood glucose rose higher in the first few hours after a meal (because there was no early peak from the lispro to help bring the blood glucose down as the food from the meal was absorbed). A few hours later the blood glucose dropped lower causing hypoglycemia because the insulin levels were now too high after all the food had been absorbed.
So when you mix fast-acting and long-acting insulins together you lose the beneficial effects of rapid peaks of insulin to cover your meals and the flat, steady, basal insulin level between meals. Even if it means having to poke yourself a few more times you get more flexibility and control of your blood glucose levels if you give your fast and long-acting insulins as separate shots.