You have to use an AI Script to turn on shared vision. It dosen't happen automatically when you set alliance status.
Example: You want Player 1 to have vision for Player 7. You use this trigger:
| Trigger |
| Players: |
| ¤ Player 1 |
| Conditions: |
| ¤ Always. |
| Actions: |
| ¤ Execute AI script 'Turn ON Shared Vision for Player 7'. |