Old post, but here's the answer ... as far as I know, there is no alternative.
If you look in .diagram in Solution Explorer, you will see that one file contains both diagrams. Since this is a .xml file, it does not have child nodes, unlike, for example, .cs files, where you can see individual methods, etc.
I usually keep the model browser open always as a tab, so it is not too stretched or stretched to open an alternative diagram.
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