I believe you are on the hook to cover both FFX and corrsponding...

I believe you are on the hook to cover both FFX and corrsponding LLL position (after demerger record date) , if you sold FFX short (before demerger record date), very similar to a company paying dividend (shorters are on the hook to pay the lender the dividend, o/w it will be no-brainer to short any stock before dividend record date, as it's guaranteed drop as the SP will always drop by the size of dividend after the record date).

Another thing to just think about, is nobody is going to lend shorters shares, if they lend out FFX (cum-LLL) shares and only get back FFX (ex-LLL). Shorters are not going to get shares to sell unless they promised to give back FFX + LLL later.