You can't expect the splitter to handle as much current as a single socket unless made really well. The average stuff that comes out of China, would never hack it, but something home made/modified and over engineered with very thick wire or a bus of solid 10 gauge wire soldered between sockets will be fine. Even so, I'd rather run it directly from the battery (fused close to battery of course) using the thickest wire practical. At least 30 amp to redcuce voltage drops on the cable run.
Graham