Combining of two or more ultra low voltage DC/DC step up converter chip's (LTC3108) output
It is unclear what behavior you are hoping for.  Do you want both LTC3108 circuits to be connected to the chargers input so either can be used, but still only one of the two at any given time? This is called 'Oring'.  The simplest (and lowest performance) way or oring parallel voltage sources is using oring diodes, which are usually just a schottky diode on the positive rail of each power supply.  In other words, you'd put a schottky in series with each of the LTC3108's outputs.  

This comes at no small cost, however.  You lose ~600mV and likely several milliwatts (the dearest commodity in any energy harvesting situation) due to those diodes, and also have to step up the voltage even more to compensate, which increases losses other places.  And the problem with oring is that it is OR-ing.  The load is powered by one thing, or the other thing.  Not both things at once.  In the case of the diodes, which ever LTC3108 can muster more voltage than the other will be the chosen one and power the circuit essentially by its lonesome, until or if the other LTC3108 becomes more wattful. Or, uh, powerful is the correct word I guess.  

If you want load sharing, which is where both LTC3108s share the work, ideally in a proportional way depending on the power available from each one, both powering the load simultaneously, you can do that too, but you'll need some help. 

Linear makes a very wide variety of "ideal diode" ICs for just about any possible application of synchronous rectifiers.  Ideal diodes are, of course, marketing speak for a mosfet and synchronous rectification controller.  It's a way to fake a diode by just turning a mosfet on and off in the same conditions a a diode would be conducting or not, yielding a pretty good approximation of a diode but with purely ohmic losses rather than normal diode loss junction loss.

For load sharing, I would recommend connecting the LTC3108s to an LTC4413 load sharing chip.  It's relatively inexpensive (a fair bit cheaper than just one LTC3108) and couldn't be simpler or easier to use.  It will act as an ORing controller when the two power inputs get to far apart in voltages, but if they are supplying similar voltage levels, the LTC4413 will actively share the load by dynamically adjusting the the voltage drop across each MOSFET to share current proportional to whatever load current each input can provide with equalized output voltages.  Basically, it just works and you don't have to worry about anything.