You need to know how it is being used and what type of 2 speed motor it is. 2 speed motors are either 2 speed 2 winding (2S2W) or 2 speed 1 winding (2S1W) type, you need to determine that first. (A "Dahlander" motor as described above is what we call a 2S1W motor in this country).
For 2S2W, the low speed can be any other normal motor speed that is available via the number of poles, because they are two completely separate windings inside of the case. So the high speed can be 2 poles (3600RPM) and the low can be 6 poles (1200RPM) or 4 poles (1800RPM) synchronous speeds.
2S1W motors are different in how they are configured and are going to be either Constant Torque, Variable Torque, or Constant HP depending on the wiring configuration. The quick way to tell is to look at the nameplate: If the Low Speed HP is 1/4th of the High Speed HP, it's Variable Torque and if the HP is 1/2 of the High speed, it's Constant Torque. If you use a VFD on either of these types, you just leave it connected in High speed and the VFD will provide constant torque at any speed. So you actually have MORE torque available at low speed if you has a VT configuration before.
If the HP is the same at both speeds, or the nameplate only provides one HP, it is Constant HP (CHP). That's where it's going to take some decision making based on what the machine does. You see CHP in a lot of metal working machine tools because some operations might require MORE torque at lower speeds as cutting tools "bite" deeper into the material. A classic case in point is a drill press where at slow speed, the bit is requiring more torque to keep it cutting as the bit digs deeper per rotation than at high speed where it takes lots more, but smaller, bites. In this kind of application, you may need to keep the 2 speed connection in play and program the VFD for the two separate loads. In that case, you buy the VFD for the highers Amp draw from the motor nameplate, then you have to have interlocking to make sure you can't change speeds without shutting the VFD off first. 2S2W motors can be like this too, so you need to know exactly what you are working with and what the machine does.
