If Adam and Eve had two sons but no daughters how did the world get populated?
The Bible is clear. The Bible states in very plain and simple language at Genesis 5:4 that "...he (Adam) became father to sons and daughters." Notice the plural "sons" and "daughters." These 'sons and daughters' would be Adam's offspring, not Seth's. Seth's offspring are mentioned in verse 7, which says "Seth lived after he begat Enos eight hundred and seven years, and begat sons and daughters." Since Adam lived for 930 years, he reasonably could have fathered many, many children. His sons, including Cain, would have married their sisters, or nieces. This would sound repulsive to us today, but consider the circumstances back then. It would have been proper at that time, since the Earth's population was relatively sparse, and the human race was to be proliferated according to God's purpose. It was only later that God made the restriction not to marry one's own sister or brother.
Genesis 5:4 The days of Adam after he fathered Seth were 800 years; and he had other sons and daughters.
As you can clearly read, the Word tells us that Adam and Eve had daughters also.
Adam didn't just have two sons, he had quite a few children. Adam's sons and daughters married each other. It seems strange to us now, but back in the day that's how they did it.
"Male and female He created them" and "be fruitful and multiply, fill the earth and subdue it" is what Genesis records before the account of both Adam and Eve being brought from the earth. Evidenced by this, we can now say the eternal God created every known living human spirit from His power, which He did, and each spirit was not born from the earth yet (but all spirits that would live, male and female, were already destined to be born on earth). Creation was complete the seventh day, and here we are still reproducing; God completed all His work and blessed the seventh day, even made it holy and commanded us to rest on this day.
There is no evidence that before the fall Adam and Eve did not have children. God commanded them to "be fruitful and multiply" so we can assume that they joyfully obeyed. We don't know how long a time was between their creation and the fall. Because sin and death were not a part of the world at that time, they could have been around for a long time, multiplying and populating the earth. Then the fall happened: this would also help explain the references to cities of people a few chapters later.
God is amazing therefore he can do anything. If he has the power to create human beings why not the power to create multiple baby's in the mother's womb. Therefore, since Adam and Eve had their children, Adam's and Eve's children could create more and more children which with the help and love of God will repopulate the earth.
When God banned Cain from Paradise he gave him a mark so that no one would kill him. This implies: a. that there were other humans outside of Paradise and b. The other humans outside of Paradise have knowledge of what Cain did to his brother Abel. It is not necessary for all of God's actions to be documented in the Bible. He could have made many more men who he did not put in Paradise The Bible tells us what God wants us to know. It in no way limits his actions.
God created Earth as a temporary abode for Humans until their time on this world was done. Skipping forward, in the Islamic stories God sent down a Woman from Paradise, a Hoori, for Abel, and a Jinn in the shape of a Woman for Kabul and believe it or not, that's how they had children.
And then they married Cousins and then second cousins and third cousins which were Halal ( Allowed ) to them until there were enough people where you couldn't tell your Cousins from a random stranger.
It's useful to remember that the stories of the old testament are steeped in the Jewish belief that they are the 'chosen' people. The idea that God made Adam and Eve in his own image and gave them a special place in the Garden of Eden does not preclude the idea that God made other humans who were not so favored. Seen this way, Adam and Eve were given favored treatment but broke the rules, and so they were tossed out into the wider world to make their way among the unchosen peoples.
According to the 32 rules of Rabbi Eliezer: The particles "et", "gam", and "af", which are superfluous indicate that something which is not explicitly stated must be regarded as included in the passage under consideration, or that some teaching is implied thereby. This rule is used by Rashi to derive that Cain had a twin sister and Abel has two twin sisters (triplets). The Bible does not mention everyone who ever lived by name. It does mention the destiny of all who live.