The dutch archaeologist and satellite tracker Dr. Marco Langbroek managed to image the two German radar satellites TerraSAR-X and TanDEM-X from ground during their formation flight in orbit with a camera. One satellite is flying in front (TerraSAR-X) while the other is following (TanDEM-X). TerraSAR-X appears slightly brighter than TanDEM-X.
(c) Dr. Marco Langbroek, Leiden, Netherlands. Zoom
A 10 second snapshot of TerraSAR (in front) and TanDEM-X (behind) on 28 November 2010. Flight direction is from right to left. TerraSAR-X is slightly brighter than TanDEM-X.
The satellite formation "provides a very nice sight for observers, especially since both satellites also produce slow, naked eye flares when the sun-satellite-observer angle is favourable" - so Langbroek.
More detail you may find: here.