I have to admit I am not a mysql 'expert' but my opinion is you would be better with 1 x 1024 but without knowing what you are running/current load its hard to say.
With 2 x 512 option apart from the fact that you will likely get an overall greater CPU share between them as its a relatively safe bet they will both be on the same physical node I don't see a big advantage if any.
But this is just based on the info provided entirely possible someone will come along and say different but not sure how they can say one way or another without further info.
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Making some assumptions, I would only add that a lot depends on your current memory usage. The easiest way to make SQL queries faster is to cache them in memory. If your VPS is memory limited then you should see some benefit from having SQL on a different VPS with a bigger free memory pool and appropriate setting changes. If you have plenty of available memory on the single VPS (not being taken by httpd, mail, av, etc) - then it's likely not worth the hassle.
The shorter version is this isn't a binary answer, you need to know your usage patterns.
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