In a lot of systems, shooting into a melee combat is either forbidden, ...
Forbidden ?
This means the designers were out off inspiration or just too lazy to make up some rules.
Targets in melee automatically get Half Hard Cover (+50 to DB) because they are moving and maneuvering and dodging and ducking and so forth.
Yes, the GM may simply state the other combatant as cover.
But it may change if you are in the rear of the target (no one in the way):
Give the target just a small DB, no damage = make a +0 Attack Roll against the opponent.*
An other solution you may find in Spell Law. Hmm ? OK, not really,
but the GM can do it like the
Displacement spell:
Give a chance that the target moves out of the arrow path. If this happens,
make an attack roll (+0 OB) against an other person (randomly choosen).
Another solution: The combatants fight just tasmanian devils:
An even Attack Roll (AR) hits PC#1, uneven PC#2.
OK, if you believe the archer could aim a little bit better, you can make each
multiple of 3 (or 5 or...) hit the wrong target.
How I would handle it: One of the above (or a different way I don't imagine now),
one time this way, next time another ...
*And one last thing: If the arrow only archieves some hits (or a low crit),
it may go farther and hit another person ...
And of course instead of a +0 OB Attack Roll, the GM may call for +20, +30 etc.