I would think the only way to play on her world would be to either use her profile, or have that switch on and her logged into her world setting it to online and then using another switch and joining that world. Having two profiles would mean that you each have your own save info so her world is separate from yours. Like having two separate filing cabinets. The seed is merely a number that's used to start (seed) the random number generator. The thing is random number generators aren't as random as you would think. So if it starts with the same seed each time you get an identical set of 'random' numbers. While horrible for things like encryption, it makes it really neat for things like Minecraft where we can have a random world that you can repeat. So using the same seed gives you an identical world, but it's in another dimension of sorts (your save filesystem) and that's why the house you built in her save isn't in your save.
Looks like I mentally swapped who is playing on who's world. Rather than trying to fix it I will just leave it as it still applies.