2nd build queue does too many negative things to the whole "opportunity cost" aspect of 4x games.
From a realism perspective I have no problem with it, and from a gameplay perspective, it works quite well (for example, in Shogun II). I think the way it's right now is fine, though.
However, if there was some manner of slider to split total city production between the two, that would be an acceptable compromise.
I do not see the point of that. Surely, no matter what your situation, you would be better off by putting 100% into one of the queues, getting whatever you need early, and then putting 100% into the other. If we had the ability to reorder items in the queue, the slider would have no purpose, unless there are penalties for skewing your production too heavily in one direction. I.e. 100 production can be 50->build + 50->train, but only 75->build / 0->train.
Too complicated. Give me the ability to reorder the queue, and I'll be happy.