Help setting up SIMS 4 for 2nd account

That's because the game looks for the saves in Documents\Electronic Arts\The Sims 4, and it finds the games from daughter 1. 

To play on the same PC I would suggest creating a new save file. If they play on the same computer that would've been the way to go. So buying another copy would've been for nothing.

If you had them play on two different computers a new copy would work best or use the same Origin account. Which would result in them not being able to play at the same time. 

You could if you really want to avoid them getting into each other's games, rename The Sims 4 folder to something else. 

So you have two folders one 'The Sims 4 daughter 1' and The Sims 4 daughter 2'. However, the game can only find the folder when it's named 'The Sims 4'.

So they would've to change the name when they start playing. 

If the first option is the best way, just start a new save game for your other daughter. I would suggest making backups every now and again, so your daughters don't accidentally mess up the save games of the other. 

http://sims4.crinrict.com/en/2014/09/backups/