Played my first game yesterday and started thinking afterwards if I had done wrong.

If I had managed to get two samples to the altar (I didn't) a shoggoth would have appeared according to my objectiv. But what happens if the investigastors reveal clue 1 or the event card (as in our case) says to reveal the objectiv. Should I then place a shoggoth on the map or would I be forced to get 2 samples first before I could place the shoggoth?

I never did manage to get the last zombie where I wanted and lost the game before I got my second sample.

You must put 2 samples on the altar to create the Shoggoth. There is no other way.

Note that the objective card says (paraphrasing) 'If there are 2 samples on the altar, reveal this objective and place the Shoggoth in the Ceremony Room'

If the objective is revealed by any other means, the Shoggoth is not placed. The Shoggoth is only placed if the objective is revealed by means of 2 samples being collected. Note that you can still summon the Shoggoth after the objective is revealed.