Chatting to two girls, getting serious with one: how to let the other one down nicely

I'm a guy who has never had a girlfriend (and is a virgin but that is outside the scope of the present discussion) so after finally deciding to do something about it, I have started seriously chatting to two girls (who are as of yet mutually unaware of each other's existence). Anyway I went on a date with one of them (despite asking the other one out as well and giving her an equal chance) and we really clicked and have been chatting quite a bit and quite seriously. Also been chatting with the other one but not quite as much. As the one I dated I've been getting on really well with and we've spoken of both wanting something serious/long term, I think its fair to all parties involved that I kind of tell the other one that I can't be with her in that way / things aren't going to head in a relationship direction. My question is: how do I let her down easily, so as to not hurt her feelings and more importantly, keep her as a backup girl in case this other one doesn't work out (because I do really like her)?

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