What do you mean? If they're both circular orbits, they'll have the same period, so they'll never collide—they'll always be on opposite sides of the Earth. In fact, this goes for any two orbits with identical periods.

Once anything collides with anything in orbit it has a slightly different trajectory. Any movement up or down will make its orbit more ellipsoidal than the original orbit. Also the atmosphere moves up and down in response to solar wind and therefore varies in both time and space. Atmospheric drag is apparently still a factor up to 600km https://www.swpc.noaa.gov/impacts/satellite-drag

Debris in a crowded orbit will eventually collide, no matter how synchronized it all was initially.

From the article you linked:

> Debris between 1 cm and 10 cm (approximately 500,000), referred to as the “lethal” population, are the most concerning as they cannot be tracked or cataloged and can cause catastrophic damage when colliding with a satellite. Objects smaller than 1 cm (approximately 135 million measuring from 1mm to 1cm, and many more smaller than 1 mm) that could disable a satellite upon impact are termed the “risk” population

There is no such thing as an identical period. It may seem identical when you compare them casually but over time the little differences add up and sooner or later the two will intersect, unless either one of the two moves to a different orbit or de-orbits. Satellite orbits are not locked together by magic, they are all independent of each other.

Perfect, well, when they have circular orbits we’ll give you a call.