[note: written before namida slipped in his reply before I posted this, but doesn't really change anything]
Lix had multiplayer for years, it even supports up to 8 players. It had some interest but overall, more people more often played singleplayer levels in Lix and in NeoLemmix compared to multiplayer in Lix. Of course, the comparison is slightly unfair since anyone can play any singleplayer level any time they want, but multiplayer needs to get people together all at a given time to work. Still, there is pretty much no one here who only ever played Lix multiplayer, but there are almost certainly people here who've been around for a while who had still only played singleplayer, and even more who might have once tried Lix multiplayer a few times and then stopped.
Given that people have played Lix multiplayer many times in the past, the interest exists. But given the likely complexity of having to program all this from scratch in NeoLemmix (Lix in contrast had always supported multiplayer from day 1, in fact multiplayer has always been a primary goal of that project and singleplayer is almost secondary), and namida's desire for a "final" version, we're likely either talking about giving up all other requested features like a 20th skill, or to wait for someone else to take over NeoLemmix development, before there is any realistic chance for 2-player mode to come about.
One additional thing to consider is that the current skillset might not be optimal for multiplayer. For example, early experience of multiplayer in Lix quickly found out some skills can be overpowered in multiplayer context, like the blocker. The batter skill in Lix was specifically invented to help counteract this imbalance. Similarly, in Lix some changes were made even to core skills specifically to avoid problems of abuse in multiplayer, for example the "basher staircase" trick cannot be done anymore in Lix (basher's stroke would instead remove terrain in one go without making any intermediate "step") as a direct result of observing it being problematic for multiplayer balance. Perhaps the situation could be a little different for NeoLemmix with its expanded set of skills, perhaps some skills can simply be avoided when designing multiplayer levels, but ultimately and more generally speaking, if we are serious about this multiplayer thing for NeoLemmix, it'll almost certainly take multiple versions to "get it right". (Just talk to Simon about Lix multiplayer, I'm sure he can gave you plenty of examples of challenges and complexities that simply don't exist for singleplayer, like dealing with effects of lag over the network.) So again, not exactly in line with someone looking at a "final" version at this time.
If you want to experience multiplayer today, but for some reason don't want to do it through Lix (maybe because the graphics are not Lemming-y enough or something, whatever), you can try SNES Lemmings over emulator. I could be mistaken but I believe a few SNES emulators can even simulate network play, although the experience might be suboptimal unless the network is consistently low in lag for both players, so perhaps not likely the case for trying to do it over the Internet (vs say within the same local network, like a computer lab at school). SNES Lemmings should have most of the multiplayer levels that's in Amiga Lemmings with minimal to no changes. If local play is an option for you, of course you can also try other versions that support multiplayer and have emulator support, including Amiga for example.