ok this may sound compilated but it's not really
1 if your in agame already then open the three dots-option manage worlds then select your household go to the three dots there manage household
2 select your household in the window that pops open another window pops open
3 select the arrows icon click on that another window pops open to select sim to split and merge you'll need to selct a houshold in the right panel to move your sim to ( if you don't see ahoushold in the roght panel click on th etimy down arrow to open the household to selct )
4 select the sim you want to move in the left panel then click on the arrow to move him/her to the right panel tick accept and that sim is moved
then close on the x or hit escape button which will be map viewand selct your household to play with again
edit to add a ps
thats the way i figured it there may be another way i don't know of yet
