How to undo system split in Sibelius 7. UPDATE: This can only be done in the full score and not in individual parts! It will however also undo the system split in the parts as well.
1. Select the bar after the split.
2. Activate Inspector.
3. In the Inspector set ”Gap before bar” to zero.
Obviously if you just made the split, you can just simply undo it. But if you made the split several moves back, this is the only way to join the bars again if you change your mind.