I have a 220V power receptacle that is wired
to a 20amp breaker.  I have a 1800W device hooked up to it.  The wire
is #12/3.

I suspect that there is a right answer and a dooable answer.  I am
looking for the latter.  Here is my question.

I have another 220V device that is 250W.  I would like to NOT run
another wire for this new receptacle, but instead to split the current
#12/3 wire into BOTH of the receptacles.  Can this be done?  what are
the risks?  What should I watch out for?  Do I have to be careful how
I hook the two hot wires up to the receptacle (do they both have to be
on the same side, etc.).

So, I know this isn't the best thing to be doing, but I am gonna do it
so can you help keep me out of trouble??  :)  thanks, david

There is no problem doing what you want. the easy way is to go to a
RV, or marine store and purchase a splitter. It plugs into the outlet
and gives you 2 plugs
Volts X Amps= Wats, so your 20 amp breaker times your 220 volt power
supply gives you 4400 wats of useable power at your receptacle and you
are only using 1800
a second device requireing 250 wats should not pose any problem Unless
 eather device   has a very great starting load like a compressor or
large motor hooked to a massive load. to be safe "FIRST"
Turn off the power at the breaker, and check with a testing device to
be sure that  the power is off.
Purchase a receptacle which is the same as the one you presintly
have(power rating and hole number and shape) and wire it with a peace
of wire the proper lingth and the same size and rating as your
exsisting wire to the nwe receptacle "Make sure it is wired exactly
like your exsisting one"
Next hook the same colored wires to their counterparts in the
exsisting recptacle IN other wards connect the red to the red and
white to white and so on
You can have as many outlets as you want and be totally safe. If you
overload the circuit the worst that will happen is no different than
any 110 wall plug which is the breaker will trip and you must un plug
something.
PS your wire should have 1 red 1 black 1 white 1 bare strand 
the red to black is 220V
The red or black to white is 110 V 
You can operate 110 components of equipmentwith a 220 receptacle that
has 4 holes in the face
red or black to neutral ,( the white wire )is 110 volte fused at 20
amps. the bare or GREEN wire is the safety GROUND What you want to do
is totally safe and normal and poses no safety problems as long as you
disconnect the power prior to doing any work and do not turn it back
on  until the task is complete and all the covers are in place and the
safety ground connected.

bidlobob:  thank you very much for your comment.  I have wired my dual
220V receptacles and it appears to be working - at least my italian
espresso machine and grinder have not blown up yet!!!

David

Most of what "bidlobob-ga" said is correct, But it is amps not volts
that can kill you if something shorts out, seeing there is no need for
more 2050 watts (1800+250) you should for your safety, change the 20
amp breaker for a 15 amp breaker, which is still more than strong
enough for your job. 15 amps X 220 volts = 3300 watts - 15% safety
factor = 2800 watts. That should keep you out of trouble.