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) :
Here’s the same chart with the fix:
The Synastry option: