Does this situation about yeast having oxygen to ferment quicker is normal?
Yes, yeast needs oxygen to reproduce. I recently read an article posted by
@stickman that was VERY interesting reading:
Can I combine the two bottles in one bottle?
Of course. In primary, it's best to have everything in one bucket, but you can split it up if necessary. In secondary, use as many vessels as necessary to limit the headspace above the wine. The smaller the volume and the smaller the surface area of wine exposed to it is, the less oxygen will be available to oxidize the fruit and/or alcohol. In secondary the yeast doesn't need to reproduce any more, so removing oxygen is good. Leave enough headspace that any foam being produced does not push through the air lock and make a mess.
For example, the reading of the filled bottle is 1040, and the reading of the half filled bottle is 1030. The average of these are 1.035 sg; Can I make this kind of calculation and combine them in one 10 liter bottle?
Your math is incorrect because you have twice as much wine at 1.040. The correct average is [ (1 bottle * 1.040) + (0.5 bottle * 1.030) ] / 1.5 bottles total = 1.037. But I see no need to bother calculating that. Of course you can combine them, in fact it is advisable to reduce the headspace. But only put into secondary once the fermentation has slowed to just bubbles (like soda in a glass), and producing no foam. We usually go by SG, but your excessive initial sugar concentration makes SG a poor indicator of yeast activity.
I see many recipes that describe the same method of fermenting wine as you are using. In other words: start with excessive sugar, ferment until yeast stops on its own or attempt to stop manually at a given SG to provide residual sweetness. In my opinion, that introduces more variables and uncertainties than necessary. As you have indicated, you have no way to know ahead of time how much sugar you are going to be left with after fermentation. You also run the risk of the wine starting to ferment again at a later time, even though you
think it's finished.
See my post above if you want a method that produces more consistent, and more controllable results.
You should spend some time in the
Skeeter Pee forum, the
Country Fruit Winemaking forum, and/or the
DangerDave's Dragon Blood Wine thread. There is a wealth of information there about making interesting and often inexpensive wine, and how and why to backsweeten properly.
Very basically, you only start with enough sugar to get the alcohol level you want and that your yeast can handle. You ferment it until dry, which makes it very easy to tell when all of the yeast is dead and the sugar is completely gone. Then you stabilize it to prevent possible refermentation, and let it clear. Take the clear wine and add as much sugar and/or other flavours to your taste. Wait a while to make sure it doesn’t start fermenting again, and then bottle it.
Note that I neglected to mention the importance of adding potassium sorbate when back-sweetening.