Doubleshot

You nock an arrow and as you say the spells short incantation and loose the arrow it transforms into two.  The second arrow can veer off and hit another target or stay true to its original coarse.  

If both arrows hit the same target:  Double the damage roll

If the arrows hit different targets: Roll separate damage die for each arrow

This spell is based off of your proficiency with the weapon used.  Use the same dice and proficiency bonuses as you would for a normal attack with the bow.