If you look into the component "chips" that make up a ram stick, you will often see a different brand (e.g. "Hynex") regardless of the branding (i.e. "Kingston")on the stick itself.  So even the same brand name and speed could have different timing or other characteristics.  As others have posted to this forum in answer to your question, you certainly could try a mixed set of "512's" to see if they work in your machine.  Thing is, in a computer timing is everything and there are several timing parameters involved in the process of memory transfers.  It might work, it might not, and as noted, performance could suffer if a matched set is not used.  
Here is a tip: check the website of the manufacturer of your RAM (Kingston, OCZ, Geil, etc.) and you should find a listing of different speeds and memory sizes.  It helps to check a reference in a book or online (in your favorite search engine)to fully understand terms such as CAS, RAS, and such and how they affect the performance of your computer.

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