AC Combiner Box for Solar Project
Splice it together in the safety switch

While Square D does not make a double-lug kit for their general-duty safety switches, what you can do is use a 14-4AWG, 3-port, insulated mechanical connector per-phase to splice the two incoming wires together with a short pigtail of 4AWG insulated copper that runs off to the lug on the switch for that phase.  That way, everything's tucked neatly inside the disconnect enclosure; this does have the downside that you can't independently disconnect individual inverters from the utility service, but for a purely grid-tied small-scale solar setup, this isn't a big deal.