sounds like your mixing units there, i'm going to assume 2mA rather than 2mV,
next step, how did you measure this 2mA? was it across the mA terminals of your multimeter or calculated from the drop across a resistor? or some other thing?
once you clear that up we should be able to work out roughly what voltage it can generate, otherwise find a known resistor, e.g. 100 or 1000 ohms, and see what voltage you get across it while operating your generator, if its greater than 0.7 then you should at-least be able to turn on the transistor,
next up combining it with another generator, yes it is possible, but probably not worth it, its the same for 2 batteries in parallel unless they are matched the more capable or charged one will be dumping charge into the lesser one trying to balance the voltage,
and finally, a transistor switches on the moment you give it more than 0.7V for general ones, then any current fed into it at that voltage is able to flow between collector and emmitor (multiplied by beta)
so if you have a transistor with a beta of 100, and you are able to feed it 2mA at 0.7V into its base, then you can use it to pass 200mA between collector and emmitor before saturating,