Turning regular door switch outlet into an outlet with USB
I've never seen 120V+USB which were splittable in the way you want.  (And we're not a shopping-aid site, so helping you find one isn't really our thing here). 

The answer is, this is a drywall job: replace the single box with a 2-gang box... keep the existing receptacle and put the USB receptacle right next to it. 

You will need to pigtail at least the hot wire, as you won't have enough screws to land supply, hot to the switch, hot to the USB receptacle, and hot to the downline.  Don't use both screws and backstabs (actually using backstabs generally is asking for trouble). 

I for one think USB receptacles are a waste of time since they're expensive and become obsolete quickly, because it takes time for them to grind through the UL listing process.   e.g. very few can charge a tablet at full power while also charging anything else, and two tablets is out of the question. Many are built around an obsolete TI controller chip which provides a 10W and a 5W USB port and you must pay attention to which is which (why????) Genuine Apple iPad charging blocks are about $8 on eBay, I just do that.