The two yellow cards printed on the assistant is a requirement to hire them. In this case, you would need to have two yellow buildings in your display. Then, on a future turn, forego placing a new building to add to your display and then attempt to hire the assistant.
If you play with the original rules, you can build and hire in the same round. So if you build the second yellow building in a round, and can afford it, you can also hire that particular assistant. In the rules v.2.0 you must choose either build or hire.