Creating differing temperature fluid gels that stay separate
This is done, and explained in the Big Fat Duck cook book, by creating a hot fluid gel and a cold fluid gel and then fitting a tight divider down the centre of the glass.  Fill both sides equally, remove the divider, serve immediately.

you can follow the link and 'look inside' to see the page which has the recipe for this on to see exactly how to do it.  search inside for 'hot and cold tea', then its on pages 274 and 275, so just follow those links.

The gelling agent used is gellan F, and making the gels requires making two parts for each and mixing them together.  The details of the recipies are not availabale at the amazon.com site, only amazon.co.uk, although you can access it from outside the uk