Existence is the coherence of the manifested like our body and the un-manifested or atma (self). We perceive existence either through the manifested or through the un-manifested. We are familiar with the first one where we distinguish between people, situations and things as our senses are capable of perceiving only the manifested. We hardly recognise the un-manifested behind the manifested as it requires going beyond the senses.
For example, when we see a person we first see whether it's a man or a woman. Then we see how well one dresses and behaves; and how influential or wealthy one is. Subsequently, we recall the good and bad memories associated with them. Our behaviour depends on the judgements that we make based on these divisions.
In this regard, Krishna says that united with the self, he sees the self in all beings and all the beings in the self, he sees the same everywhere (6.29). This is the path of going beyond the five senses. Metaphorically, it is like digging a well where the senses see sand, stones and gravel however no water at the beginning of the digging, but invariably water appears.
This verse gives a paradigm shift of looking at the un-manifested first and coming to the manifested subsequently. It's the realisation that behind every person or thing (manifested) the same un-manifested exists which Krishna said 'seeing the same everywhere'. It's like two fighting fruits of a tree realising that the same trunk feeds them and they are a part of one big tree. Then the entire thing becomes a drama played on the grand stage of the un-manifested.
Certainly, as indicated by Krishna earlier, this needs constant practice with determination to see the self in all beings and all the beings in the self.