As with any good debate I have found two varying arguments, I can’t find the original link and soruce from T-Nation though so these are random articles from various nutrition sites
so here goes, first is Against mixing at certain times and second is to keep them together
(1)WHEY TOO FAST: SLOWING DOWN ABSORPTION
All that being said, whey protein seems to be the most popular choice on the market today. A lot of people combine their whey isolates with milk to slow down the absorption. Sometimes they mix their whey in water for a fast anabolic response after training and milk at other times. Some mix it in milk at all times or water at all times. The evidence is very varied. However it stands to reason that a whey isolate in water followed an hour later by a slow releasing protein is a good idea. Whey isolate in water in the morning may also be beneficial but avoided at night or at least added with milk before bed to slow the absorption down a little. A better choice protein like cottage cheese or micellar undenatured casein would probably serve a better choice before bed. Even a whey concentrate would enter the system slower and seems a better choice at times other than after training. A further argument then takes place where athletes such as bodybuilders whom consume 6 or more meals a day, often more frequently then every three hours and sometimes even more frequently than every two hours, don’t require a slower releasing protein and should go for the one with the best makeup of amino acids and highest in BCAA’s - being whey protein. The protein wars are typcally opinion based, or commercially fueled, with very little major concern for too much scientific fact. It is this fact that probably leads many health enthusiasts and trainers to actually experiment to find what they feel works best for them.
Milk Proteins: Whey and Casein Work Better Left Together!
Some things are best left together. Like milk proteins.
Milk is a mixture of two major proteins, whey and casein. Whey protein is frequently sold as a stand-alone supplement or used as the sole protein source in meal replacements. Not surprisingly, however, research is beginning to suggest that leaving casein and whey together (as in a “protein blend”) may have multiple benefits for your body.
Whey is commonly described as a “fast” protein, meaning it is quickly digested and absorbed. Casein is considered “slow” by comparison. Thus, by supplementing your diet with a blend of casein and whey, presumably you can receive the benefits of both “speeds” of protein (i.e., rapid yet sustained delivery of protein building blocks to your growing muscles).
There may be other benefits to combining your whey and casein. Whey protein is a source of a number of biologically active peptides (i.e., short chains of amino acids). Some of these peptides may provide healthful properties (i.e., lower blood pressure, improve mood). Casein helps to preserve the functions of such peptides. This effect appears to be unique to casein.
Casein seems to block the breakdown of whey peptides in your intestine. It can increase the lifetime of some peptides by over 15 times. That gives the peptides a greater opportunity to do your body good. In the same way, casein may also render your whey protein more useful to your muscles by slowing down its digestion and absorption.
To sum it up, a protein blend supplement containing casein and whey may give you not only the bodybuilding benefits of “slow” and “fast” proteins, but also a host of other healthful properties that might be absent when consuming either protein on its own.