This is new research that could take a stem cell and make it into a sperm or an egg. So, it's plausible that we could take the stem cell from one man and make that into an egg and fertilize the egg with the sperm from the other man. This can then be placed inside of a woman and she can give birth to the baby that is not genetically hers. The baby would be genetically derived from the two men. This is not a possibility yet, but could be in just a few years. Would this be haram?
If anyone's sperm goes inside the woman (except her husband) then this is haram. As touching, talking and even gazing any Na-Mehram is haram let alone putting sperms in her (by any means).
So, as to your question,"
If two men could have a baby (e.g. via stem cells), would it be haram?". Yes, it would obviously Haram.