Online Multiplayer Cross-Platform Sidescrolling Platformer Games


So, I have been badly wanting to play some games with a (new to gaming) friend of mine, turns out I couldn't find many games that were easy (Sidescrolling/ simple controls), cross platform (I have Windows, she has a Mac) and have online multiplayer option.

I tried introducing her to gaming with Trine 2, that has Online Multiplayer, is Cross-Platform and a Sidescroller, but the multiplayer issues with that game are just terrible. Also, she found it slightly difficult and I couldn't really explain the game to her. Does anybody know any games I can play? Free and paid, both are okay. Thanks :)