So the most common copper twisted pair cable is 4-pair cable. Four pairs are used for 1 Gbps connection speed, while 100 Mbps connection uses only two pairs of four. So you can use single cable for either single 1Gbps or two different 100 Mbps connections. But it is decision you should make when you building your network for first time, since it will take some considerable time to rewire sockets on both ends if needed.
Which makes me ask: there must be some bright fellas who noticed possible benefits of being able to plug extra jack when needed (while sacrificing speed, of course)?
So is there RJ-45 socket, which you can switch between Single and Dual simply by replacing its front or pressing button?