Combining Voices

Help! I imported a Musescore piece into SP successfully. I then added a staff above the import to write a solo melody. I used "copy/paste" parts of the original to write the solo melody. Some how I ended up with some of the score in Voice 1 and some parts in Voice 2. My mind says they should be combined into one voice. Is that correct thinking? (I could not find anything in the manual on what to do.)

Would someone please set my thinking straight and tell me how to combine the voices. Also, are there some pages in the manual that explain how voices work and when to use them to their full advantage. Thanks a bunch!

Jimmy