Is it possible to use the eco tech tree as a tycoon player, and vice versa? If it is possible, how do you unlock the respective tech trees?
If you're playing an "endless" game, when you get your citizens to tier 5 (Executives), you'll be able to purchase the rights to construct the opposite faction's starting buildings. (The rights for Tech buildings are unlocked at tier 3 for both)
In most of the scripted missions (the campaign, etc) you are only given access to the opposing faction's buildings if/when it is scripted.