You can use the same Fillet tool to do 2d fillets.

But it will only work on independent curves.

What probably happened to you is that you selected 2 line curves but they were not independent lines, they were edge curves that belong to a plane. Right now it won't do a 2D fillet in that situation, when edges are selected it wants to do a solid edge fillet which won't produce any result since those edges were not joined to any other surface.

I'll see if I can tune this up so that it will do a regular curve fillet if two edges that are unattached to anything are selected. It won't automatically trim the surface in this case (maybe some other day in the future...) , but I should be able to make it generate the arc.

- Michael