Steps to repro:

1. Open Unity;
2. Open attached project and 'test' scene;
3. Notice that there are 2 active Inspectors in the scene;
4. Select Terrain gameobject in the Hierarchy view;
5. Select "Raise/Lower terrain" tab in the first (upper) Inspector;
6. Start painting mountain with the first brush (circle with smooth edges);
7. Notice that height with correct brush is added;
8. Select "Raise/Lower terrain" tab in another Inspector (lower one);
9. Choose different brush (i.e. Star);
10. Paint Height on terrain.

Actual result:
Brush is still painting initially selected form (circle), however another brush (from the second Inspector) is rendered in Scene View.
Only one Inspector is taken into account when painting on Terrain.
See attached gif video.

Expected result:
- Brush that was selected in the last chosen active Inspector should be rendered and correctly painted on terrain;
- All active Terrain Inspectors should be updated instantly when latest one was updated (changed brush form, texture etc).

Notes:
- Reproducible in OSX 10.12.1 and Windows 10;
- Reproducible in 5.5.0b11, 5.5.0b10, 5.4.2p2, 5.3.6p2.