Ligatures for two voices in unison

Can you explain what you mean by a "ligature"? The score you posted looks good to me - it seems very clearly notated in terms of the use of the multiple voices. What specifically are you wanting to see differently? Are you trying to convert this to a more Renaissance-era style of notation?

I suppose you want to add a slur in measure 1. Hold shift to select the chord, then hold shift + arrow right to extend the selection. Add the slur.
(alternative for selection: press shift and make the selection with the left mouse button)

If you cannot select the 2nd voice in unison (starting with F or G note)
a) Click on the previous note on the 2nd voice (in this example: note C)
b) Press the right arrow key (twice for G note) on the keyboard (currently the 2nd voice is selected, but selection is not visible)
c) After this point: If you want to make slur, Press "S" key on the keyboard
...

(french....) I suppose you want to have in measure 1 a slur above and one also under the notes . You have the 2 slurs but ther are "superposed" if you click on this first slur and drag , you will see the 2 slurs . You can drag one , under the notes and click X for changing the form, or, in beginning with the "double" slur, click X, so one of the slurs go down , and drag it where you want.

Ah, I see, "ligature" = "slur". And the problem is how to create one in voice 2 given that it shares notes with vocie one, making it hard to select the voice 2 notes only.

Some good answers already, but here are my suggestions:

1) Easiest may be to is to remove the existing slur and add them both at once. Click the slur and press Delete to delete it. Now select the range of notes you want slurred - eg, click the first note, Shift+click the last - and press "S" (or double click the slur icon in the palette). Bam, the slur automatically gets added in both voices at once.

Or

2) If you don't want to remove the existing slur, that's OK. Make the selection as before but use the View / Selection Filter to exclude Voice 1. Now when you press "S" or double click the slur icon, the slur is added to voice 2 only. Be sure to re-include voice 1 when you are done!

Or

3) Click the first note where you want the slur, press Ctrl+Up to raise the pitch an octave, thus moving it out of the way to expose the voice 2 note underneath. Do the same for the last note. Now click the first voice 2, ctrl+click the last, press "S" or double-click the slur icon,.

Or

4) Click the closest voice 2 note where you can get a clear shot - in this case, the "C" right before the unison passage - then press "right" twice to get to the note where you want the slur to start. Press "S" to start a slur there, then Shift+right four times to extend it.

Or

5) If the notes hadn't already been entered, you could have pressed "S" right after entering the first note, then entered the rest of the notes, then pressed "S" again when done to terminate the slur.