It's actually not so hard to code. You just need a love and a dom path, two variables. And in each scene with her renpy just has to know wich path you are on.
So it's not difficult but it doubles the amount of work for this char and if you want to see all scences you have to play the game two times. The main problem would be thatif you play only one route you get less scenes per update then otherwise:
Dev makes 10 scenes (8+2 with dualchar) per update and needs 3 weeks to do so.
Dualpath: still 10 scenes in 3 weeks but 6 scenes for normal chars and 2 for lovepath and 2 for dompath
-> If you decide to only play the dom path you get 2 scenes less then otherwise.
"I had to wait 3 weeks for only 20min gameplay? This dev sucks and should work harder!" :-D
But yeah then you could chose to have that girl ineracting with you the way you prefer, so better gamequality....
Dev will have to chose.