If you use Z-wave as the basis you can do this easily using direct association. All you need is z-wave controller that is capable of setting up the association, and you don’t need the controller after that point.
If you just want to keep both switches in sync that you need two switches that support direct association. If you are looking to add another switch, Cooper/Eaton has a batter controller z-wave switch you can stick on the wall, or you can extend your existing box to add a second powered z-wave switch to assoctiate back to the other switch. You can use a cheap controller to setup the association, like a minimote but I have found this has become iffy if their minimote can setup the association correctly.
It is also important to note that you must have a Common/Neutral line any place you are going to put a z-wave switch.
If you want to just get something up immediately that needs no controller at all and no neutral.
Look at the Caseta Switch with Remote. Caseta is made by Lutron and is an excellent product. The Remote just sticks to the wall and is battery powered, it can be mounted (with a kit) if you extend your box like a normal decore switch over the jbox so looks exactly like the Caseta switch. You can associate the remote and switch directly without having to have their hub, so don’t worry about adding the hub. If you do add a hub later then you can connect Caseta to other systems that support Caseta (Wink supports Caseta directly without the Caseta Hub), most other you can add a plug in to talk to the Caseta Hub that then gives you control of the lights.