Be careful, you can't just take multiple battery packs and wire them together. The pack with the lower charge will try to charge the other one, and it'll try to do it at the highest current that its protection circuitry will allow. This could get dangerous and it's certainly not good for either pack.
If you fully charge each pack on the same charger, then hook them together, then tie them together with a Y adapter or whatever you're going to use, and then always treat them as a single pack after that, it should work OK. But don't take a half charged pack and plug it in with a fully charged pack, or anything like that.
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Work: the 8 hours that separates bike rides.