Download some DJ software like BPM Studio, or PCDJ. They have free trial versions. You can trim the songs anyway you like, beatmatch, cue and blend the together just like a pro DJ. It takes some practice though, and the BPM's (beats per minute) have to be somewhat close or one song will sound either too fast or too slow. Or you could use some linear music production software like Sony Acid Pro, or Avid ProTools, then render the songs into a project and cut and paste, blend, whatever......If you just want to blend the beginning of one song to the end of another and have constant non-stop music, DJ software is the way to go. I use Alcatech's BPM Studio. Type it in google there's a free trial version.