Editing/merging two mp3 files. Can someone help me, please?

I want to make a custom ringtone for my dad by editing together two different mp3 files. I have the files but I don’t have editing software and this isn’t something I need all the time so I don’t want to purchase a program.

The one I did find online just doesn’t work well.

Can anyone direct me to a free site to use? Or, if any lovely jelly were willing to do this for me, I would be forever grateful and reward with 10,000 internets and a whole lotta lurve!