The question is old, but I'll try to answer it anyway...
The three connections (1x plug, 2x socket) are NOT all connected 1:1 as with a "double plug". The adapter splits the 4 existing wire pairs (1/2, 3/6, 5/4, 7/8) from the plug to the two sockets as follows:
1/2 --> A 1/2
3/6 --> A 3/6
5/4 --> B 1/2
7/8 --> B 3/6
(The assignment is a guess, I do not own this adapter. However, I do have a
Value RJ45 T-Adapter (UTP, Cat. 5e) in use.)
With this you can use an 8-wire cable (four wire pairs) for 2x 100MBit/s (requires only two wire pairs). However, you always need such an adapter on both sides!
If you want to extend the cable: There are double couplings for this purpose, e.g.
Roline RJ45 coupling (S/STP, Cat. 6).
For "sharing" a connection with several devices: A switch.