connecting things in parallel (not in series) is the route you'll need to take, be that connecting more solar panels to give you the same volts in but more current or connecting resistors across the circuit.
maybe connecting a single 2ohm resistor across from the + input (assume 3.5v in) to the + output (5v measured out) of the device you've bought will give you close to your desired results of 2amp out at 5 volt.
no idea on what total effect that will have on the voltage step up though or if you'll have heat issues/failures because of the added resistor.
like i said been way to long since i've messed with electronics to offer any reliable detailed advice to solve your problem but can't see why adding the linked resistor won't solve your problem if my memory isn't playing tricks with me, which it seems to do more often as i get older.
link that will help you understand how parallel/series circuits work. :- http://www.gcsescience.com/pe20.htm
Edited by terry1966, 14 February 2016 - 12:23 PM.