couldn't find the answer anywhere, anyone know?

Maybe you can find something useful here? In any case, it should be easier to set up using a source port, rather than trying to get DOSBox to work for multiplayer. Alternatively, you could grab the Megaton edition on Steam, which has built-in multiplayer support, but then you'd have to convince your friend to do the same. :)

If I wasn't currently stuck with an extremely crappy internet connection, I'd look further into it myself. I've never played Duke3D outside of the single player missions, but it looks like fun! Good luck getting it to work.

https://wiki.eduke32.com/wiki/Multiplayer

And here's the 'latest' version for eduke: https://gitlab.com/mossj32/EDuke32-OldMP_Maintenance/tags/