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I played a maelstrom game where I had 2 cards with similar requirements.

The first card stated that I got a victory point for destroying an enemy unit in close combat or by shooting. If I destroyed units in both close combat and by shooting then I received D3 points. The second card stated that I received a VP for destroying a unit in close combat.

During my turn I destroy one unit during the shooting phase and another during assault. So, I applied the first card (D3 points) AND the second card (1 point).

My question is: was that legal?

Yes. Nothing states that each "action" can resolve only a single card.

It isnt a detachment being part of another detachment issue (e.g. 2 troops being present in both a formation and a cAD) where this is explicitly denied.

If I remember correctly, away from the book, the Rules do address what happens in these situations.
Both are scored.