How to..um...put a girl "on hold"?

mrock
I recently started dating a girl that I like. It's going well and I suspect will be at it for a while. After a few dates with her, another girl that I like gave me her phone number, obviously with some kind of relationship in mind.

I texted the girl that gave me her phone number, and said we should hang out some time, but have never actually scheduled anything. It's been about two weeks since then, and I saw her once afterward in person and she is still interested.

I really like both of them, but feel more loyalty towards the first girl. I'm tempted to take the other girl out on a date or two, but I don't want to ruin things with the first. I just want a way to keep the second girl as an option if/when things don't work out. I don't want to make her feel like I'm uninterested. Gotta avoid the friend zone too.

Ideas?
How to..um...put a girl "on hold"?
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