Agreed with all above. If you are unsure, give the leader an option in the party settings to "allow ally/enemy signs from party members". They will most commonly be used by people during a Free Company event, and most Free Companies will be in voice chat anyway.

The final alternative is give the party members the ability is to make the leader's signs a larger or more obvious appearance (different colour or a larger sign), that way there will be no confusion when 2 targets are specified.