The House of Wonder says you can spend 2 gold to get either a cleric or a wizard. If I have 4 gold, can I get 2 of those adventurers in one turn?
No you cannot spend more than 2 Gold, yes you get 2 Adventurers.
The rules (page 5) under Assign Agent require that a building has an unoccupied action space to place your Agent. Immediately below that, the parts of a Basic Building are diagramed, showing what an Action Space looks like. On page 8, the House of Wonder is detailed, showing a single Action Space and the following text:
So, yes you get 2 Adventurers with spending 2 Gold. You cannot get 4 Adventurers, because the Action Space would be occupied by your first agent.