You should be able to do that with a DAW if you don't mind using a computer or laptop as a go-between. In fact, you can layer more than two voices on the receiving keyboard, or create multiple splits, transpose to create harmonies, etc. For instance:
Keyboard 1 --> Channel 1 --> DAW Track 1 --> Notes 0 - 42 --> Transpose +12 Semitones --> Keyboard 2 --> Channel 1
Keyboard 1 --> Channel 1 --> DAW Track 2 --> Notes 0 - 42 --> Transpose +12 Semitones --> Keyboard 2 --> Channel 2 (for a layered sound in the left-hand split area)
Keyboard 1 --> Channel 1 --> DAW Track 3 --> Notes 43 - 85 --> Transpose +0 Semitones --> Keyboard 2 --> Channel 3
Keyboard 1 --> Channel 1 --> DAW Track 4 --> Notes 43 - 85 --> Transpose +0 Semitones --> Keyboard 2 --> Channel 4 (for a layered sound in the middle split area)
Keyboard 1 --> Channel 1 --> DAW Track 5 --> Notes 43 - 85 --> Transpose +7 Semitones --> Keyboard 2 --> Channel 4 (to give the second layer a "perfect fifth" harmony)
Keyboard 1 --> Channel 1 --> DAW Track 6 --> Notes 86 - 127 --> Transpose -12 Semitones --> Keyboard 2 --> Channel 5 (for an extra voice in the right-hand split area)
You could actually do all of that using just one track in Mixcraft, but using a different track for each outgoing channel would let you temporarily disable that channel in the DAW if you wished. Also, you'll want the DAW to send the Bank Select, Program Change, and other sound-controller messages (e.g., Reverb Depth, Chorus Depth, Attack Time, Release Time, LPF Cutoff Frequency, LPF Resonance Level, etc.) for each outgoing channel.
PS -- If you want something that doesn't involve a computer or laptop as a go-between, I believe the term you'll want to search for may be "MIDI router."