You are saying that since the internal battery resistance will be halved, the charging current will double. Also, it looks like there is a slight chance that the cells will get off-balance.
It sounds like it'll be easier to get a 5v to 12v voltage booster and feed it from a mobile power unit via usb.
Something like this, perhpas:
http://www.ebay.com/itm/1PCS-LM2577-...50852080428%26
http://www.ebay.com/itm/LM2577-LM257...50852080428%26
http://www.ebay.com/itm/DC-DC-Switch...item4abc2a1bad
I'm not sure which one would be best though