i have the same thing, started as 6 frame nuc, then last month before feeding i gave them another 6 frame nuc above it, so total there are 12 frames. i think i'd leave them as is now, as they are likely to be better this way (bees goes up, less so sideways) so to avoid isolation starvation, so felt its better to overwinter in 2 x 6 frame nucs stacked on top of one another, than 1x 12 frame hive where we intervene with their current setup..
they are still being fed and are taking on 1l to 2kg stronger syrup now.