Overhead Belgium some air corridors are present managing traffic west-east bound or north-south bound and both vice versa. Sometimes you can see contrails of aircraft cross each other, the airplanes self might closing ready for a mid air collision, see also this link, but it's nothing more an than an optic illusion from the terrestrial observer. Minimal safe vertical distance is 1000 ft and from the ground airplanes at cruising level (around 36000 ft) tend to look having the same dimensions.

On November 10 2011, a Tupolev 154 of the Republic of Slovakia was flying somewhat behind a Boeing 737 of Bmibaby, but it had a slightly faster speed. Both west bound, their flight paths had not exactly the same direction, they would cross at a certain moment each other. The 737 was flying some 2000 ft lower, but from my point of view they completely overlapped each other on a thrilling moment, all digitalized with telescopic aid.