Dimmer switch plus a regular switch
One Dimmer to Rule Them All

In general, no matter how many control locations one has in a lighting branch, one can only have one dimmer for that branch, at a single control location, namely either the "line" or "load" end of the switch arrangement.  The other control locations receive switches, wired in a traveler-system multi-way configuration to the dimmer, provided the dimmer provides what is known as a "3-way" or "traveler" terminal, which indicates that the internal switch in the dimmer is really a 3-way switch instead of being an ordinary single-pole switch.

Digital technology changes the rules

However, modern digital wallbox lighting controllers, such as the Lutron Maestro and Leviton Vizia, change the rules of the game.  Instead of being stuck with the limitations imposed by mechanical switches, these systems use communicating remotes mounted at the other locations.  These remotes talk to a compatible master dimmer over a proprietary powerline-communications protocol, often implemented using one of the traveler wires to power the remotes and the other to aid in communications.  Furthermore, this may allow the master dimmer (which does all the heavy lifting of dimming the lights) to be placed at a variety of locations in the circuit.

A commercial aside

If you have seen commercial lighting control systems, you will notice that they do not rely on wallbox mounted lighting controls.  Instead, they use low-voltage controls that mount to the wall.  These controls then talk over a wired interface to a central lighting control panel that performs the dimming work.  This is more costly and infrastructure-heavy than wallbox controls, which is why it is not commonly used in all but the highest-end residental work, but does provide a great deal of control flexibility and also handles higher power than wallbox controls can manage.