I've seen Nate put in Mills and Miller at the same time twice now. He did it against Minnesota, and he also did it one or two games back. Both times it has hurt us. And the reason it hurts us is clear.
Let me be clear. I am a huge fan of both Mills and Miller. Miller is one of the better, and most underrated point guards in the league. And he is unselfish, often getting his team mates great looks. And Patty is improving every game, and gives the team a whole different look when he is on the court.
First let's have a look at the strengths and weaknesses of our two point guards. Let's start with Miller. His strengths are obvious. His size and ability to post up other point guards is a huge plus. Against smaller point guards, Miller can take them to the post and overpower them. Also, Miller is extremely reliable from mid-range or closer with his jump shot. Most importantly in my view, is his court vision. The amount of easy baskets he sets up is phenomenal. He and Camby both give Aldridge a lot of easy shots to put in (though this has lessened somewhat as every team now looks for it). Now for the weaknesses. The first is that Miller brings a slow tempo to the game. This really hurts our players who thrive in the open court, as opposed to the half court. Though to be fair, Miller appears to be more willing to push the pace over the last couple of weeks (at least, in my opinion). Another weakness is Miller's inability to take long range shots, which means that he finds it difficult to space the floor, since defenders will drop right off him, and often crowd the paint. An important point here, and I don't consider this a stregth or weakness, is that Miller is a point guard that needs a lot of the ball to be effective, which is why Roy made it so clear that he did not enjoy playing with him.
Now for Mills. His biggest weakness is his size. There is nothing he can really do about this, and it doesn't seem fair to mention it, but the fact is that Mills lack of size is a notable weakness, mainly on defence. He has done well to cover this flaw so far, but against a point guard like Miller, for example, he would get taken to the post on pretty well every play, and the Blazers would be forced to double team Mills' opponent, or basically just concede the basket. His size also means that Mills finds it hard to drive into the paint and draw fouls, because defenders can just hold their arms straight up in the air and make Mills shoot low percentage rainbow shots over them. The other issues Mills has, that don't relate to his size, mainly relate to inexperience. For example he doesn't provide the alley oop pass nearly as often as he should. He hits Aldridge when he's posting up, but not when he's flying through and has a bit of space on his opponent. I'm not concerned about this kind of thing though, because he is improving rapidly, with Miller there to play mentor. One of the big pluses of Mills' game is his long range shot. The percentages don't actually show what a good shot Mills is. This is for two reasons. One, early on in his career, when he was just seeing garbage minutes, he was a forcing a lot of shots that weren't there, and those misses continue to anchor his FG percentages. The second reason is that on botched plays when the shot clock is almost up, Mills is quite often the option to lob up the desperation shot. Normaly these shots are well defended, and generally they don't fall. And the second advantage of Mills is that he is able to space the floor, because he is regarded as a threat from long-range, which gives everybody else more room to work. Mills also loves to push the pace, which helps a few of the guys get good looks on the open court. Once Mils learns to run the fast break more effectively, this will be even more of an advantage. But again here, neither an advantage nor a disadvantage, Mills needs a lot of the ball to be effective.
So you can probably see a lot of oposites in the strengths and weaknesses of our point guards, which is why they work well in giving opponents different looks. However the one big thing they have in common is needing a lot of the ball.
When they are on court together, Miller naturally takes the lead. He does most of the ball handling. Meanwhile Mills just sits at the 3 point line, and can be covered by a guy just sitting in the paint and rushing Mills if he gets the ball. So in essence, it is like you are playing one man down, because when Miller is on the court, all of the advantages Mills brings to the court are nulified. And it becomes harder for Miller as well, because he is basically 4 on 5. So both of them play worse and it destroys team momentum. It is no coincidence that Minnesota started showing signs of life at the exact same time Mills and Miller shared the court. I know Nate is only doing this with big leads, presumably to see if he can make it work. It may work every once and a while, but I think it is a losing formula.
That was unnecessarily long-winded, I know, but I wanted to talk about why I think Mills and Miller work so well together (just not at the same time), since I have seen so many comments that favour one over the other.