I sometimes need 10kmah and sometimes 20kmah and would prefer not buy 2 separate battery packs. Money is no object. Is there a cell phone USB charger that is chainable?
If I understood you correctly, a "cell phone USB charger" is a power bank. If that's right, then as far as I am aware, you have two options:
Buy a 20Ah charger that has multiple batteries inside, wired in parallel. When you need 10Ah, take out half the batteries; when you need 20Ah, put them back in. Example: 4 batteries inside. 3 batteries inside. 8 batteries inside (but you must take out 4 at a time).
Buy two 10Ah power banks, and a two-to-one USB cable, which would allow to use both of them at the same time if that's what you need. You can put them both in a single case, to make sure they stay as a unit. Using both simultaneously is particularly useful if your device uses a lot of power.