Roger MacBride Allen's The Torch of Honour and Rogue Powers
These were originally published as two separate books, but subsequently combined into Allies and Aliens.
The Torch of Honour sees a married couple - Mac and Joz - on a survey mission as the only people that can insert into a system that has recently been invaded by the Guardians, a bunch of right-wing human fanatics. The Guardians have made a space-borne counterattack infeasible, but Mac is able to insert with plans for a teleport device that will allow a League taskforce to bypass the orbital defences and teleport onto the planet to free it. The final phase of the book deals with assaulting and finally boarding a lighter-than-air spacecraft/mothership to disarm the system that is preventing relief from outside.
The second book, Rogue Powers, deals with events at the Guardians' homeworld and their dealings with an alien race with Lamarckian biology. Kidnapped League scientists who are being closely supervised by the Guardians are being used to make first contact with the aliens, who are on a planet orbiting the companion star in a binary system with the Guardians' planet's star. The Guardians use the aliens' technology to develop metal-eating missiles which are used in an attack against the main League carrier fleet, which Joz is on board at the time. However, a fanatically nihilistic group of the aliens have decided that they want to exterminate the Guardians, so that at the end of the book, the League has to save the Guardians from extermination by an alien bio-weapon that is targeting the Guardian "homeworld".
One of the memorable - to me - elements of Rogue Powers was how a chance comment about "new stars" made by a Guardian defector to a fighter pilot in the second book allowed the unusual astronomical aspects of the Guardians' binary star system to be deduced by an astronomer, finally allowing the League to locate the Guardian system.
