That was going to be my next question, can you stack them front-to-back just sitting on the floor without lifting them onto the (bolted to the concrete wall) mounting plate? Doesn't look like they are in danger of tipping over, though wiring them might be difficult without some kind of custom conduit box...
Outside of conduit at least; or with the conduit box, They wire much easier as the -/+ are on the same side. If you put them in front of one another