How long would Fondant last
Firstly, sorry, I thought there were two sensible replies.
How long? That depends on lots of things.
How much? 12.5kg should be enough, but may not.
Why? Because we don't know what the winter is going to be like, the strength of your colony, the size of your beetainer and the insulation/ventilation arrangements, the site topography, how much stores are actually available already and whether you intend feeding strong syrup now, and lastly, when the winter might actually arrive.
For instance I saw bees taking in pollen, at least - on the winter solstice and Christmas Eve last year.
Let us know how strong or weak they are. I am of the opinion that uniting may be the best option for you but can not be sure, of course. How strong is the other colony? I have learned from experience that weak colonies are more of a problem than their worth and you may be at risk of losing both if the winter is severe.
One strong colony is worth more than two weak ones, the weak colony may be poor due to a failing queen and not survive anyway - in which case you may not need much fondant at all!
So you will need to monitor the usage and add more if/as necesssary.
Regards, RAB