Q:

As followed by past simple or past continuous

Recently I had to sit a test and one exercise was particularly tricky: all inflected forms of the verb in brackets were removed and I had to fill in with the correct tense. 
Personally, I don't think such exercise proves much when it comes to evaluating your command of the language as - sometimes - more than one option is correct and it may all depend on your personal interpretation of the text, especially considering the verbs can be modal or negative! In theory it sound easy but in practice... it would be difficult even in my own mother tongue. 
I was able to find the text online: 
"In 1880, it was slow going for Joe Juneau and Richard Harris as they ......... (search) for gold with the help of Native guides. After climbing mountains, forging streams and facing countless difficulties, they found nuggets "as large as beans."
My entry was "were searching" which, needless to say, was considered incorrect. The correct answer was "searched".
Is there a big difference between the two forms (in this specific context)? If so, do you have any tips so I won't make the same mistake again in the future (since I am considering a resit. 

A:

The past continuous/progressive with as establishes an ongoing background action/state which is somehow interrupted by the more topical action/state in the main clause:

One day as they were searching along the sides of a beaver pond, my mother stepped to the edge of the water and was bitten by a large poisonous snake.  — Dean W. Brown, The Journal of Samuel G. Horton, 2010.
As they were searching for their carriage, they bumped into Jean-Paul Sartre, who was traveling on the same train, though he was destined for a different conference.  — Frances Stonor Saunders, The Cultural Cold War: The CIA and the World of Arts and Letters, 2013.
As they were searching, Joey noticed a shiny rock in an underground cave.  — Romena Jonas, Golden Head and Silver Head, 2014, 20.

Keeping both clauses in the simple past means that the two actions/states occurred more or less simultaneously:

The three slowed down, their footsteps gently padding along the shore as they searched the mist for the shape of their boat. — Robert J. Skead, Patriots, Redcoats and Spies, 2015.
So firmly held was this belief that Spaniards and Portuguese were willing to confront great hardships and horrifying dangers as they searched for gold and silver. — Kendall Brown, A History of Mining in Latin America, 2012.
Partners danced and changed partners as they searched for their true match. Women were nonchalant as they held drinks in their hand. — Harvey Rothenstein, The Big Picture and the Adventures of Life, 2007, 95.

