What I do whenever I think up a new idea that I really love while I'm writing a story is I just write it down, just map out the basic ideas. Like plot, characters, conflicts, etc. Then you finish your current story. And afterwards you look back and well what I feel is a flood of inspiration and your brain has already subconciously added to your ideas whilst you were finishing up the other one. I would strongly advise you not to do two stories at once because then I find the quality of work plummets. Sorry if this was unhelpful or confusing!!
Best of luck, LAURA
Ok I get what you are saying :D
I think I shall do that :)
~Transmission Exploder xoxo