Well you do enter all your income on the same 1040 return. You enter W2 income under Wages which goes to 1040 line 1 and 1099 self employment income on schedule C which goes to 1040 schedule 1 line 3 then goes to 1040 line 8.
To enter a W2
Click on Wages and Income tab at the top
Then Wages and Salaries
Wages and Salaries – Start (you might have to click on Show More on right)