I can think of some situations where dual-wielding weapons could be feasible/useful, considering the accuracy penalty. Covering a door from the side, perhaps 2-3 spaces away, with reaction fire, should an alien come barging through. I don't think it should be restricted to identical weapons, but similarly-classed weapons: pistols with pistols, rifles with rifles, RPGs with RPGs (though that would probably only make sense for a barn broadside that needs breaking).
I would, offhand, throw out that dual-firing options could bridge similar single-fire modes from both weapons into one option, costing 75% of each bridged mode (i.e. firing a single shot from one costing 8 and the other costing 6 together would cost 11, with an accuracy penalty).
I think the dual-wielding concept, however, does need to be addressed for the late-game, with the inclusion of powered armors. From the looks of things, human infantry armor may reach the point of having multiple weapon systems installed for simultaneous use, with computer targeting and recoil management to make accuracy manageable. Now, loading four primary weapons onto such a powered-infantry suit might be unreasonable, from a believability and codeability standpoint, but two would be explainable. Some method of firing two compatible weapons together would be needed.