According to Exodus (taken from the translation by Rabbi Aryeh Kaplan), the Israelites never actually saw God, nor did they witness any major miracles. The only thing they saw was the mountain being engulfed in thunder and lightning, and then smoke. God warns Moses, "Go back down and warn the people that they must not cross the bounday in order to see the devine, because this will cause many to die."(Exodus 19:21)
The revelation that occours at Mt. Sinai was a revelation by Moses alone. He then relayed the information given to him by God to the people. That was his first trip up the mountain
The second time he ascends the mountain, he remains in waiting for seven days before God finally speaks to him. During this time, the Israelites whitness what could possibly be seen as a true miracle: "To the Israelites, the appearance of God's glory was like a devouring flame." (Exodus 24:17) However, this passage does not state that they really "saw" the flame, in empirical terms. It is stated more as a metaphor for how the Israelites viewed the experience of Moses going up a mountain and seemingly communicating with God.
After his seven day wait for God, Moses is called upon and "went into the cloud, and climbed to the mountain top." (Exodus 24:18) He remains there for forty days and forty nights before returning to the people with tablets in hand.
Now it seems to me that forty days is plenty of time to fabricate a couple of tablets. The Israelites did not really witness anything aside from some lightning and thunder and Moses going up and down the mountain a couple of times. So the revelation at Sinai is still a personal revelation of Moses. God never revealed himself to more than one person at a time in the Torah.