I wasn’t sure from your description if you wanted to leave the two original switches each controlling one light and add a third device that would control both or if you were just talking about each of the two switches controlling both lights.
If you just want both switches to control both lights, @ritchierich mentioned you just replace each of those switches with any smart switch that works with SmartThings and then use the official smartlighting feature as he described.
Adding a third device
If you want to leave the two switches each controlling a separate light and add a third switch that will control two lights, you can do that as well. All three have to be smart devices that can talk to the SmartThings hub. There are a number of battery operated switches/buttons which could work for the third switch, you could put that anywhere you wanted.
or just use switches in the two original positions, but choose devices with double tap functionality
There are also some mains-powered smart switches which have a “double tap” feature so that, for example, pressing the top of the switch once would turn on the original light and pressing it twice would turn on both lights. Then you only need two smart devices, not three, but the usage would not be as obvious to guests since you have to know to double tap the switch to turn on both lights at the same time.
The switch choices available do vary depending on which region you are in.
If you are in the US, the homeseer switches and the newest GE switches have the double tap feature built in.
If you are in the UK, popp has a similar switch but in a UK design. 