Fuelling multi-day riding (carb loading/replacement)
Summing up the comments, advice received elsewhere, and experience having tried it:


Hydration is just as important.  You don't want to start the second day even slightly dehyrated (which you wouldn't notice sat at a desk, so may nopt be expecting if you're used to single-day rides).  Start rehydrating as soon as you finish the first ride, even if you think you don't need it.
A proper dinner on the night in between rides (carbs and protein) and a proper breakfast (carbs).  If you're starting a hard effort early, you may have your own approach to breakfast.
Do try to keep carbs topped up on the first day, whether this is by proper food or gels.  The choice is likely to be a matter of preference and ride type.
Carry more gels/sweets/energy drink than you think you'll need on the second day, and/or be prepared to stop and buy something.