So shop around for a switch mode boost converter? There are plenty of schematics available on the web, try looking for some of those first come up with a few of your own ideas first then we have something to help with. You should have no problems finding dedicated IC's for boosting from/to those voltage levels, or even discrete circuits.
Keep in mind it's not safe to charge lithium batteries in parallel, they need to be charged individually, lithium's low internal resistance and the slight variation between cells will cause one to overcharge (possibly fatally) while the other one will be under charged which will lead to even more wasted power as the stronger of the two cells tries to balance the weaker one. NiMH's and NiCads were much easier to ballance because they could handle a slight trickle charge all the time which would do the balancing, such a scheme does not work for lithium's because the batteries will gradually be destroyed if they're trickle charged the lithium metal actually plates out onto the anode or cathode (I forget which)