Are there actually plans for implementing a network engine into duality in future?
No plans so far. Duality is plugin-based for a reason, and with a core team that small (me), it's really important to not try to do everything at once. Networking code doesn't need to be hardwired into the core.
Do you believe it is easier to implement 2D multiplayer in Unity? I just want to make sure, since multiplayer is a feature I'm interested in. I'm alright with learning game mechanics, as you can see them in action, but am bad with more technical stuff. I never have implemented multiplayer on my own, so need to be sure it is possible before I stick to Duality.