This is a little late on the response, but...
The first stage of fermentation requires oxygen. The second stage requires that exposure to oxygen be as close to fully eliminated as is possible.
Typically per instructions, when the fermentation is taking place in the primary, it is meant to be exposed to air (for oxygen). At about the time the fermentation is to be moved to secondary, it is time to eliminate exposure to air.
If you have a fully sealable fermentor, which also can take an air lock, you can do both stages in the same fermentor container. You seal the fermentor and add the air lock when the SG gets to the SG mentioned in the instructions, where the wine would normally be transferred to secondary.
However, what was said above about over access to gross lees and to skins is still an issue, if fully fermented in the primary fermentor. Some wines can stay on the lees and on skins longer than others. Just do your research for the particular wine you are fermenting at the time.