In OS-X, is there a way to open multiple windows of TeXmaker, each for viewing and editing different (or same) files?
In recent versions (4.2+) it should be possible to launch a new instance of
texmaker using the option
E.g. from command-line
texmaker -n whatever.tex
The whole story
From the source files package,
./utilities/CHANGELOG.txt we can read
version 1.7.1 -> 1.8: texmaker can now be only started once per user : inverse search will no more launch a new instance of texmaker
version 4.1.1 -> 4.2 : with the new option "-n" for the command line, a new instance of Texmaker can be launched (will break the synchronization with the pdf viewers and others feautures)
The Puzzle / Documentation bug [?]
texmaker file [-master] [-line xx] [-insert foo] [-n]
-noption a new instance of Texmaker is launched (will break synchronization with the PDF viewers).
But in the same section of the manual on the official site  it is reported a different syntax:
texmaker file [-master] [-line xx] [-insert foo]
BTW in the documentation files of the source package  downloadable from the official page, the
-n option is listed in the manual... unfortunately I had not the opportunity to compile or to install the last version, but it should work for each system.
I'm currently using the 5.0.2 version under Ubuntu.
Even if not reported by man, if I use the
-n option it opens a new instance of texmaker.
My best alternative is to use another tex editor (such as the basic texWorks which comes with the mikTex download)
EDIT As mentioned since OS-X does not use miktek another solution is to download, install, and use TeXstudio to view two documents at once. Again it's a work around but it's simple and quick.
Not an exact answer but just made my comment to answer to keep the thread answered:
I don't think Multiple Texmaker windows could be opened(as suggested by Jubobs) because all
.tex files are opened inside one window only with drop down menu selection of each of them. Hence an alternative approach is to switch between source viewer--> comparing with other
.tex and pdf viewer--> view output of