That is a best case scenario. you make some money for two weeks, and hopefully leave on a good note. In asnwer to their question, I would recommend telling them gently but honestly. You never know in this business when you just might go back there. Leaving on a high note is both recommended and smart.
When I left my former employer, they tried to get me to come back for a long time, knowing how hard it is to start a business. Never once did I go after their clinets, which left an open door back at any time. They now do sub work for me, but if all hell broke loose, I am confident I could still go back. When I turned in my truck, it had all of their tooling in it. They actually called me surprised, half thinking I would take it to start my biz, the other half pizzed off I didnt, cause then they had to clean it out and inventory the tooling.