Over the next month Simon learned to shape-shift. The key was sex. If Simon imagined having sex as a woman that he had touched he was able to take her form. It helped greatly if Monica took the form of that woman's partner. Initially the shifts were very short lived, 30 seconds or a minute, after which he would revert back to his now regular form of Monica's twin. Whenever he got close to orgasm he couldn't hold the shift.
After a couple of weeks he found he could stay in Susie's (his mum) form for an hour, so long as Monica was in the form of Graeme his stepdad. This was of course only on the proviso that she was turned on by foreplay but didn't orgasm. She needed the sexual tension to change but couldn't lose control. Quickly he learned to do the same in other female forms.
When he managed to stay as Liz Smith's form for an orgasm, as Monica (in Dean Smith's form) stimulated him with her tongue. And not revert back that was a great breakthrough. After that he quickly learned to orgasm in any female form and not revert back to the 'Monica' body.
Simon's progress learning to change into a male form was not going so well. By the end of the month he could be his mother for an hour even if not being sexually stimulated but his own male form he could only hold for less than a minute.
With all the practice of changing into male forms in order to have sex with Simon, Monica had become a master of shape-shifting. She had no trouble holding the "Simon' body all day. Her one problem was that whenever she slept she would still revert to her true female form.
Over the month they had essentially swapped lives. During the day Monica would become 'Simon' and Simon would pretend to be 'Monica'. One thing the couple did was to move into a unit of their own. It was just too dangerous for them to live at Monica's parent's house once they returned.
Simon now made a very convincing 'Monica' he could apply his own makeup and had essentially taken over her part-time job as a sales assistant in a department store, while Monica became 'Simon' a university student. However his desire to be able to hold his original form and be Simon once more, seemed a distant dream.