Class 90 electrics in tandem.
 

There is a photo in the gallery showing two class 90 locos on a freight concist.

https://www.railwayforum.net/gallery...date=lastvisit

Two questions.

1) Why would any freight movement need that much horsepower, or is it in case one breaks down ?

2) I seem to remember reading that the current draw of two class 90s was too much for the overhead system in some locations, so the combination was effectively banned.

There could be a few reasons for double heading Philip ,
More power, so the locos aren't working flat out.
Insurance if one breaks down.
Extra brake force to control the train.
Second loco could be on a balancing move , so rather than run it light loco and pay for track access charges , crew etc, attach it onto another train.

You are correct about class 90's double heading but as far as I recall it's only on the Great Eastern mainline.
The West Coast overheads are rated higher for " juice extraction" so double heading is the norm .

Cheers John