I have a 240 inline fan that can be speed controlled, would it be safe to run the 2 hots to 2 seperate 120v outlets that are both on seperate breakers? or could I run the two 240v hot wires to the one hot wire of 120 with just less power? the fan will be used 3 hours per week
The fan may or may not work on 120V, it's impossible to say without more information. Best not to risk it. It might be possible to rewire the fan to run on 120V--many appliance motors are dual-voltage, depending on how the windings are connected--but again, it's impossible to say without more information.
Running a 240V device from two separate 120V branch circuits is not safe or compliant with electrical codes in the US. Your country may be different on the code front, but it's still not safe.
Consider what happens when you have one (male) plug connected to one circuit, and the other (male) plug not plugged into anything. The unconnected male plug will be connected to 120V from the other circuit through the fan motor. The motor is almost certainly low enough in impedance that you now have exposed contacts sitting at 120V with the ability to apply a lethal amount of current to anyone who comes into contact with them.
So don't do it. Have an electrician run a proper 240V circuit or get a 120V fan.
AC consumer fan motors that are designed to be speed controlled can usually be speed controlled by voltage reduction. However, you are asking about a 50 Hz, 240 V fan that should be supplied with 288 volts for full magnetic excitation with 60 Hz. That makes 120 volts at 60 Hz which would be 42% of full voltage. A fan and motor combination running at 42% of rated voltage is not likely to provide much air flow. In addition, the motor may stall or run erratically. If it runs smoothly and provides enough air flow, it is probably pk to run it on 120 V, 60 Hz.
Another question here mentioned a product with safety features that provides a means of using two outlets to get 240 volts. As Transistor mentioned, being on two circuit breakers is not sufficient, the two circuits must be derived from different phases.
One option that you can try is to use a series capacitor with a value picked such that the capacitor resonates with the motor inductance at the power-line frequency. This has worked very well for me when re-purposing large box-type fans removed from surplus equipment. Fans are rated for 220 Vac / 60 Hz and I see about 180 - 190 Vac RMS across the fan winding.
Be sure to use a non-polarized capacitor(s) with a voltage rating of at least 200V.
If I recall correctly, capacitor value for my particular fans was about 12uF.
What you do is to grab an assortment of small capacitors and simply try various capacitor values while monitoring both motor voltage and air flow. Add capacitors in parallel to build up towards the desired value. The voltage will peak with a particular amount of capacitance. As you increase the capacitance beyond that value, the voltage will start to drop again.
The fans that I performed this trick on have been working reliably since the early '80s.