If you look at silver haired maid in that scene, she's also holding a bundle that's clearly a baby. They're really not hiding it, they're really pushing the foreshadowing. My thinking for why Empress Frau separated her daughters is because the Emporer said he only wanted 1 kid.
I think the reason why he wants 1 child, he didn't want their child to have a similar situation like him and his older brother (possible the older brother risks his life for the sake of his reputation)
Yeah, he said that he didn't want his children to fight over the throne
When the Empress gave birth to twins, she trusts one of her maid (silver hair) to keep Freya safe since the Emperor only wants 1 child (though for sure he would be happy about the twins, there must be a reason why the Empress separated them). We got to see the other maid (Shina, the head maid) holding Lucia from the Emperor's flashback. While the other maid (silver hair) is the one taking care of Freya as seen to Freya's flashbacks, and maybe Shina didn't knew that the Empress gave birth to twins, only the silver hair maid knew.
Obviously we know they're twins because of the pendants (one at Lucia/Freya's commoner house, one for the Emperor), and how the Emperor called Freya's name in chapter 1.
At the same time, Empress Frau might knew that the current Empress might do something if she knew about Frau having twins. We know Frau is headstrong, so she must've plan early to keep her girls okay, but let's see what will happen