You can record multiple tracks of virtual instruments at the same time. By default, Mixcraft will listen to all MIDI channels (Any Channel) for each track.
However, if you click the little arrow on the Arm button you will be able to choose the MIDI input and MIDI channel that the track listens to.
For example, if you have two keyboards and want to record each keyboard on a different track, try the following. Set keyboard 1 to send on MIDI channel 1 and set keyboard 2 to send on MIDI channel 2. Now click the Arm button on track 1 and select All Input Devices to Channel 1. Choose Channel 2 for track 2. You are ready to record.
Alternatively, you could split your MIDI keyboard into two sections. You could make the first section or range of notes go to MIDI channel 1 and the second range of notes go to MIDI channel 2. This would have the same effect as two keyboards. You will need to consult the manual for your MIDI keyboard to see if this is possible, though.