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Hmm the main girder would be the front one of a line of them behind and not simply supported by the single girder at the front, naturally I do not show these extra supports
Yes, that is true. In your case, there would be multiple girders parallel to the outer girders, possibly constructed as multiple spans next to each other. You may or may not see the girders between the parallel tracks, depending on how it was constructed. There would also be cross-member girders spanning between the parallel main girders.

The challenge with this of course, is that you have pointwork on your bridge which would have to support trains transitioning between parallel bridge spans.
In practice, the permanent way engineer would try to avoid placing turnouts on a bridge if possible, because such layouts would impact the construction (and cost) of the bridge. However, if there was a requirement that it had to be done, the engineers involved would need to engineer a viable solution at a reasonable cost.
This is the sort of thing that when it is 'done right' on a model layout, it 'looks right' and adds to the overall appearance of accuracy - makes it look like the modeller 'modelled what they see' rather than what they 'think they see'.
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