Is it possible when buying a card that allows you to place a disk in boomtown as a bonus to also do a separate boomtown action if you have the metals?? I know this would usually break the rule of Only placing one disk in boomtown per turn, but wasent sure if it was ok as the card provided a bonus..
That's fine. It doesn't break the rules.
You did an investment card whose action allowed you to place to boomtown.
You may still place a disc directly to boomtown on your turn as well (as long as you have the metals per the usual rules regarding boomtown)