wiring two 110 v outlets from one 220 v outlet
Ok so that connection socket is a NEMA 14-30 and is intended for a dryer.  

You say you are plugging 240V tools into it now.  Are these tools sized for a 30A circuit?  Or are they smaller draw (&lt;4800W) and you just cheater-corded them into this receptacle? That does not provide safe overcurrent protection.  

If all your 240V power tools are >3840 watts

Stay with the NEMA 14-30 receptacle.   

You can also install another 14-30 in parallel with it.  You can't pull them both together over 30A, but this spares you from constantly plugging and unplugging the fragile sockets.  

Now there are only two legit ways to split this out for 120V loads: make it a subpanel, or install a PSU like those intended to distribute power to a rack of servers.   The subpanel option is maybe $60-80 and the PSU option probably $200. 

If all your 240V loads are 4800 watts or less

You can use the subpanel or PSU option to split up the 30A supply.  

Or you should be changing your breaker to 20A and changing that fat receptacle to a NEMA 6-20 type (which they make in duplex receptacles if you look widely enough).  Your appliances should get NEMA 6-15 plugs, or 6-20 if they exceed 2880 watts. 

Once done, you can also connect 120V receptacles (the common NEMA 5-15/20)  in the fashion of a "multi-wire branch circuit" or MWBC.  

Those get wired as neutral to neutral, and line to one of the 240V legs.   It's also possible to split a double receptacle to power each socket off an opposite 240V leg.  This would allow you to put two 240V sockets and two opposite-pole 120V sockets in that same 2-gang junction box. 

Take care to wire all 120V sockets so that if you removed the socket, that would not break the neutral for anything downstream.  This is a Code requirement. 

Pencil in a budget of $25 for all this stuff, unless your breaker is GFCI/AFCI, then more like $100. 

Lastly, common sense applies: just because you can plug many things in, does not mean you can run them all at once.  

One more thing.  Are you sure you're saving any money going with 120V heaters?  Cheap foreign-made $20 heaterfans are complete pieces of junk that might last a season.   A 2000W, 240V baseboard heater is only $50, and they're a heck of a lot better value, not least because they'll last 20 years.



Continuous loads must be derated 20%, which becomes 1440W, 1920W, 2880W or 3840W respectively.