I have a very basic question.  A system with two 120VAC pressure switches needs to activate a 3-way air solenoid that would open the valve if either switch activates.  Can I just splice these wires together and attach them to the solenoid?

Yes, and consider using screw terminals instead of splices if practical.

If the solenoid has a 120 Vac coil, it would be correct to series the pressure switch contacts that open to de-energize the solenoid.

Consider the pressure shelf positions.  If one switch is a low pressure switch, wire the common and normally open switch.  For the low trip the contact is open while sitting on the shelf thus below the trip condition.  The normally open contacts close after sufficient pressure exists.  If the other switch is a high pressure switch, you would wire the common and normally closed contact.  On the shelf the high pressure switch would not exceed the setpoint thus would be closed.  For a high pressure trip the contacts open after the pressure increases above setpoint.  Also, if the trips must be clear to start something, you could need a startup time delay with a contact across the low pressure switch contact that permits increasing to satisfactory pressure before applying the low pressure contact.

We also like additional contacts that generate alarms that indicate why the solenoid tripped.  Most of us prefer transmitters that can indicate the pressure thus provide indication of the measurement device failure.  Pressure switch failure can be undetected for a very long time without anyone being aware.

Thank your JL.  This is an interesting application of a steam tunnel between two buildings that each have their own boiler. The valve with the two switches is normally closed on this connecting steam line.  Basically, if either boiler fails (thus a low pressure switch on either side of the valve), they want the valve to open to allow steam from one building to the other.  I was thinking of using a limit switch on the valve, they already know if the boiler is down, but this would give them positive indication that the valve is opened (or closed).

Valve position switches or a position transmitter can also provide status of the valve.