Thanks. That means you are intending to take the Apivar out after 7 ish weeks. Although the instructions say 6 - 10 weeks, my own experience is that Apivar can be quite slow to get all the mites, it is best to leave for the full 10 weeks. Therefore, if you add another brood box or whatever way you make the split, all brood boxes from now till almost end of October should have 2 strips per box.
Nothing worse than cheapskating on the treatment then finding down the track your hive is just not performing and you miss a lot of potential honey crop.
No need to worry about contamination of the current honey box. When you do your split, that box will be the second box on one of the hives, and at seasons end should be left on the hive for winter feed.