In adding harmonies it is possible to use more than one middle voice. The following example uses an upper middle voice, which is the harmony closest to the melody note. For example, in bar 1 the A note is chosen as harmony note because it is closer to the melody than C or E.
You will notice that in bar 4 the same note (E) is used in both of the middle voice arrangements. It is quite acceptable in harmony to overlap the voices in this manner. Also notice the `symbol next to the E note in bar 17, which indicates that the note is to be played on the second string.