I might try that for the watermelon. I have read reviews that state it is VERY sweet as-is so adding some less sweet white might be a good idea.
The sweetness of the watermelon will come from the F-pack. There are several ways to reduce sweetness:
1. Add some of the F-pack when you start the wine, so the sugar in it ferments. Note that WE stated there is sorbate in the F-packs, so it's likely the Orchard Breezin' does as well. This may not prevent the fermentation, but may affect it.
2. Don't use all the F-pack. When backsweetening, add half the pack and taste. Keep adding until you think it needs
just a bit more, then stop. If you don't use it all, put the remainder in the freezer to use for something else in the future. [I have no idea what else you might use it for.]
3. Make a larger batch of base wine. The more wine you have, the less sweet the F-pack will make it. For this reason (among others), do not short the water.
4. Make a neutral-ish white kit separately. When ready to bottle the watermelon, put all the watermelon base in a primary, add the F-pack. Add the white kit, a few cups at a time, stirring well in between additions, and taste. When you get the level of sweetness you like, stop and bottle. The plus of this method is you'll have at least a few more bottles of wine to enjoy.
The remaining white can be used to do the same for the peach, and any remainder can be used to top up other wines or simply enjoyed on its own.