Can metal framed single pane windows be sandwiched together to make an effective double pane?
Basically, you can make a double-pane window, but so far as thermal insulating properties go, you are highly unlikely to achive anything of value. External storm windows serve to keep rain/snow off the window proper but do almost nothing to insulate (unless of course the interior window leaks like crazy and the storm window doesn't).
Sealed double-pane insulating windows are specifically designed with fluid dynamics in mind.  Aside from filling the gap with a dry inert gas, the spacing is carefully selected.  A very thin spacing will allow too much direct conduction heat loss thru the gas.  Too large a spacing will allow internal vortices to form, leading to large convection heat loss.  

In sum, unless you dig up the proper spacing specs and are able to match them in your construction, you won't get much additional thermal insulation. You may get added sound insulation, if that matters.