Venus and Mercury can be opposite the Sun in a composite midpoint chart
John Townley in his book Composite Charts states:
...if the two natal Suns are nearly opposite, you may find that the near midpoints of Mercury and Venus may wind up in conflict (such as in opposition!).
Here’s an example composite chart (comprising of ‘two made-up’ natal charts) :
Composite chart: Position of Venus – opposite Sun, problem
Here’s the same chart with the fix:
Composite chart with Venus position fixed
The Synastry option:
Synastry Options with composite midpoint Venus Mercury choice