Minecraft wiki entry for Nether_portal, associated text: Portals can be placed together in a tunnel-like fashion, though it appears as if the third portal is lit as the first two in a row mimic glass. If more than six portals are connected, the inner portals are completely invisible while in the portal tunnel, however, the particle effects can still be seen throughout. These connected portals also share the 4-second countdown until teleportation, so as long as the player is within a connected portal, they are sent to another dimension.
The player cannot open their inventory or the chat window while standing in an active portal, and any other GUI (such as that of a chest or villager) is immediately closed when opened.
If 2 portals intersect, and the player lights a fire in the intersection (i.e., in both portals at once) only the portal that lies within the x-axis is activated. Lighting any other block activates whichever portal it is in.
There is a splash referencing the nether portal. It says "Slow acting portals!".