Yes, both models have 2xDIMM according to Dell configurations page and Service manuals. Previous gen 7390 had only one slot, but was a bit thinner. Quite incredible in this size laptop. 7490 had already 2xDIMM.
Dell is jumping from the Latitude 7390 and 7490 to the Latitude 7300 and 7400, respectively, for the 2019 refresh. These new models will be slightly thinner than before with Whiskey Lake-U CPUs and vPro in contrast to the Kaby Lake-R SKUs on last year's models.