Cupcakes, our system has some similarities to yours, despite the differences I posted about earlier!
Everyone has to do the language they started with in Yr 7, but can choose to do a second language for GCSE - at the point they have only done about 6 months of the second language vs 3 years for the first.
This is exactly what my daughter is doing - she has done French for nearly 3 years and has chosen to do German, which she did between Sept last year and Easter this year, so just over 6 months.
Her language teachers fully expect her to do just as well at German GCSE as at French, despite the disparity. Those who take up the second language will have one after-school lesson a week from this Sept, to enable them to 'catch up'. However they also benefit from exceptionally small classes, because not so many take up the second language.
In addition, the second language is only done by those who are fairly high achievers, hardworking and able in the first foreign language - the school would not allow others to do this option, and to be honest I can't see anyone without a flair for languages doing so anyway. In the past, those who have done 2 languages do just as well, if not better, at the second as they do in the first, and many have gone on to do both languages at A level.
I have no qualms about this at all, I think once you have a strong grounding in one language it really does become easier to pick up another. I found this myself, I had a year and a half less German than French up to O level and A level, and still always did better at German, and liked it more, right through to degree level.
So don't despair yet!