Tube formation
How a connection between two bacteria might be established?
We do not know much about connections between bacteria. Let's suppose that the sugar envelope of one bacterium is somehow connected to another. Then inside this envelope, for several reasons, the inner pressure of bacteria can locally rise. This pressure can cause the formation of bulges growing from each bacteria inside the envelope. Their tubes could meet and even touch each other on the extremities.
In this situation, the phospholipids on the ends of the tubes are rather unstable. Their hydrophobic parts are more exposed to water because of the high curvature on the extremities. So the energy-preferable state is to fuse.
In this case, there is no specific protein involved to do the fusion, so this explanation seems to be the compatible with reality.
In further paragraphs we will propose a version of the process happening after the membrane fusion.