Hi NoY, each voucher should be good for the round-trip, per person (i.e. if you are travelling with someone else one-way you cannot apply the voucher for two one-ways at 50% each instead, although the cost would be the same). Once a phone agent even suggested that the voucher is valid for more than two segments (e.g. XXX-AUH-YYY and back, four segments total), but I have not had a chance to try it. Maybe you will have better luck!

*ETA: thanks for the confirmation sabdhabi! That's very good to know!