Bearing in mind lots and lots of people take higher doses, and the normal release without issue, it's a very personal thing where your tolerance levels lie. Literally, the only thing to do would be to try it, as an almost halfway house. Although it's less that the 2 x 500mgr daily dose, it is more in a single dose, so who knows.
The only consolation to the experimentation is Metformin is not an expensive drug (usually !), so provided you don't buy (I recall you're not in UK) a huge amount, it shouldnot be too expensive an experiment.
If your Doctor will prescribe it, why not give it a go?