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The studio/gallery crew  will almost certainly do breakfast and the One, it’s actually not that much of a stretch 
It’s unlikely a breakfast presenter will do the One more because of the brand identity rather than hours. 
There may well be a new face, a current face may move, someone, somewhere will have a plan…. Hopefully…

Whilst there isn’t currently much overlap between the breakfast and bulletin presenters (although Tina Daheley does occasionally present both) there has in the past been much more overlap. Louise Minchin, Sian Williams, Kate Silverton and Simon McCoy regularly appeared on both whilst Breakfast was in London and Nicholas Owen was a pretty regular relief presenter on both in the early Salford years.