It might sound crazy... but what if the 2 expansions were made to interact seamlessly?
So one player who has entrenchment has Starbases etc, the other player who has diplomacy has spies etc, but they dont have what the other player has, so i can fortify my systems and bring some major hurting on your fleets and systems, but you can sabotage my empire and idn whatever else IC has planned...
i realise its VERY riskay, but if it works?
Alternatively... they may need to make it that you need Entrenchment for Diplomacy... it makes sense, really, why would you buy one X-pack but not another?