Do I benefit my professors by publishing papers with them?
Would I be acting out of line asking Professor K to conduct this research with me with hopes of publishing a paper even though he won’t be my adviser and graduate me from my master’s program?


You wouldn't be out of line, but you should check that Professor&nbsp;L is OK with you working with&nbsp;K, too. L's biggest concern is most likely to be whether or not you have time to work on both projects as well as going to classes and doing whatever else is required of you as part of your studies.

If you're not already exceeding expectations on your first project, it's unlikely that you'll be able to make good contributions to two projects simultaneously.  Also, bear in mind that, as a master's student, you almost certainly haven't yet developed an accurate judgement of what would or would not be publishable.


  Are there any benefits for him from working with me, or am I just being selfish by wanting to do both projects? Note that he’s not yet tenured (will this help on his track?).


Yes, there are benefits to everybody. Academics are primarily judged in terms of things like their research output, their ability to attract funding (which, in turn, depends on their research output), and their work with graduate students. Working with you directly helps him on two of those and indirectly on the third, so you're not being selfish at all.

And working with smart, enthusiastic, hard-working people is fun.