Hi Lueng Jam,,
Your situation is so similar to mine mate its eery. I was with a failrly well known UK Wing Chun association and trained privately with one of their instructors for three years, helped him open huis class, taught for him when he wasn't there as well as assisting with beginners.
About a year in I also started learning under my current teacher, as he was much closer to the source of Wing Chun. For a while it worked out OK , my treacher was cool about things, we shared ideas and i trained in both camps. After a while, I realised what i was learning under my current sifu made much more sense (IMHO) and i decided to leave the association. I think my teacher knew it was coming and we both agreed when my last lesson would be.
At the time we both said we'd meet up for chi sao etc but very quickly the politics becomes involved. When teh last class came around i was disappointed in that he never thanked me for the committment i'd shown, and did not wish me the bset of luck in my new school - it was all a bit cold-heated. Like you said , your teacher becomes a close friend, not just a sifu. I mde many attempts to keep in contact with him, dropped in for a cuppa etc but in all honesty he has shown no interest in remaining friends etc so ive just called it quits.
Unfortunately it is frowned upon by many associations if you train outside your school / lineage which i think is total b***cks If i ever i was told i couldn't train with someone Id leave the association and just teach independently.
Hope it all works out for you - PM me if you need some advice.
Regards, Stuart
Ip Ching Ving Tsun in South Wales - www.swanseavingtsun.com