Today the standard voltages are 120 and 240. The measured voltages at individual premises may and will vary slightly.
If fed from a double-pole breaker that is common-trip and shuts off both poles with one motion of the hand, the installation you envision is permissible.

Not really a good idea. Most devices that require 240 volts also require dedicated circuits. Also the 120 volt receptacles require GFCI. If you do it with 12/3 you will require each receptacle to be GFCI or a GFCI breaker.

It is called a multiwire branch circuit (AKA an 'Edison circuit) and is acceptable if you meet the handle ties rules bus driver listed.

Even older versions of the code required a common trip (handle tie) if a device yoke (duplex receptacle strap) had both legs present (upper receptacle on one leg, lower on the other).

The regular 120/240 service uses the same method.
The neutral only carries the mismatch current between the two legs, and if they are exactly 180 degrees out the same (including phase shift and harmonics) there is zero neutral current.

Without seeing your home it's hard to say but typically a garage would be served by a single circuit. With that kind of setup, you couldn't get 240v. 240v is derived from two hot wires on opposite poles. Even if you have opposite legs available, remember that you are limited by the existing wiring. If you have, say, 14 ga. wire then you can only use a 15 amp breaker (and a 20 amp breaker for 12 ga. wire). If you plan on using any high draw tools, like a welder, you are going to need a new circuit with the appropriate sized wire and breaker.

You might be better served by installing a small sub-panel in your garage for your 240v circuits and new 120v circuits. Remember too, that current code requires 120v circuits to be GFCI protected in a garage.

Thanks for the responses. The garage has a 150A sub panel or I wouldn't have asked the question, frankly. It already has a 20A 240V branch properly breakered to a single outlet.

This garage is a part-time wood shop. Currently have two staionary tools capable of operating on 120 or 240 volt. One is wired for each at present because that's how they arrived.

Not sure about the logic of 240V devices requiring individual circuits. Nothing magic about the voltage and the big rule is not to exceed the capacity of whatever circuit you might have. I was thinking to install 12 gauge wire and the saw and jointer each have 15a plugs, and draw around 5.5 amps each. I don't envision running both at once. Am I missing something?

My intent in putting them on 240 is to add versatility to the shop and impose some automatic discipline in separating medium draw devices from, say, the radio and battery chargers by putting the power tools on the power tool circuit. Then if I had the 120 also available then I would tend toward using those outlets for lower-draw tools also, like grinder, scroll saw.

I could see installing a GFCI in each box as they're not that expensive. Seems to come down to price of individual GFCI's versus the wiring effort and the price of wiring.

Since the garage itself is a work in progress I think I'll

1. Think it over for now

2. Add outlets to the existing 120V circuits where appropriate

3. Extend the 240V circuit to a couple convenient spots, and if there is not already a 120V outlet nearby, add a 120V GFCI receptacle

4. Make up a 240V extension cord for the time being.

How's that sound?

Using a 20 amp multi-wire circuit for higher draw 120 V tools works fine.

It gives you two separate 20 amp circuits on each duplex receptacle if you want it.

It allows the use of a larger router and a vacuum for chip collection without overloading, all you have to do is make sure they are plugged in to the different legs.

In my shop all lower receptacles in a duplex are on one leg, all uppers on the other.

Any large stationary tool that can be converted to 240 V usually should be.
It reduces the length of start up current and this reduces temperature in the motor.
The power consumed is not any different but with half the current it is far easier to deliver nominal voltage without going to larger conductors.

Separting devices is a good idea. Your original plan won't do that. They would both be running on the same circuit. The 120 volt devices would use half (one hot and neutral)and the 240 volt devices would use the whole circuit(both hots). That is why I suggest separate 240 circuit not a multiwire circuit with both 120 and 240 devices.

If your machines are wired for 120 now you will need to change the plugs when you convert them to 240.
There is no advantage to going 240 volts in this case. The only advantage is if the device is large current draw eg 30 amps @ 120volts, it would be half the draw on 240 volts (15 amp) and would allow use of smaller cables and breakers to install. Since your machines are 5 amps they would still need 15 amp circuits but now you would need a double pole breaker and special 240 volt receptacles and plugs.

Brickeye and Bus_driver - thanks for the assistance.