Good deal...I'm interested to hear how it goes.
For future reference, when pitching on a yeast cake keep in mind the type of yeast and the beer color of both of the beer that is producing the yeast cake and of the beer/wort you plan to pitch onto that cake afterward.
You always want to start with a lighter color/lower OG beer to produce the yeast cake (first beer) and pitch a darker/higher OG beer onto the cake (second beer).
Also consider beer styles. Here, you are going to pitch a Nut brown ale onto a Wit beer yeast cake. I know you are in salvage mode for the Nut brown, but that is probably not the right kind of yeast for that beer. Belgian Wits are made with some fairly specific yeast strains and produce some fairly distinct flavor/aromas. Not the kind of flavors/aromas you would necessarily want in that Nut Brown.
Pitching onto yeast cakes works best when you are either producing multiple batches of the same styles of beer, or the yeast is a neutral yeast that can be used in multiple styles.
Good luck!