With a power requirement of 24 V and 6 Watts it doesn't require much current. However, I would go with a 24 VDC supply referenced to ground as I would worry about the DIO. The DI is designed for 24 VDC inputs and they should (I believe) share the power common. I would find a DIN Rail mounted 24 VDC single ended supply for it. Not saying the split won't work, only that I would not do it personally.
Wire their inputs in parallel and connect to your +12 volt high amp source.
Wire their outputs in series to give you 30 volts at 1 amp.
Your solenoid consumes 6 watts, this is 0.25 amps at 24 volts.
Sticking a dropping resistor in series with the solenoid to eat that extra 6 volts you have:
R = E/I = (30-24)/0.25 = 6/0.25 = 24 ohms.
It dissipates P = EI power = 6*0.25 = 1.5 watts. To keep it cool derate that wattage by a factor of 2.5 or 3. You'll need a 24 ohm, 5 watt resistor. This is from the same place that sells the converters and will work great for this:
**broken link removed**
Be sure to stick a commutating (or freewheeling, or back emf) diode like a 1N400x across the coil to eat the coil's spike when it shuts off.
Plan"B":
If the - 12 volt power supply you have can supply that 0.25 amps (+ 12 supply is usually good for beaucoup amps) AND the + and - share the same common, then just wire your solenoid to the + output and to the - output.
Don't forget the diode.
I am aware of that. It should be the device I linked to in my later post. I was wondering where Jaguarjoe came up with a solenoid.
Anyway, because of the reasons I mentioned I would use a single ended 24 VDC supply as I mentioned and not use the split supply. Pretty much as I mentioned, based on what you have.
There is no diode or need for a diode. The diode would have been used if there was a solenoid. Don't worry about it. Just some minor, humorous confusion.