What is the best strategy to use if I have to compare the ideas of 2 texts at the exam?

The best strategy would be to firstly read the whole text in order to get the main idea. Then read each paragraph of the text aiming to get the main idea behind it. Very often the first sentence of a paragraph (or sometimes the last) may convey the idea of the paragraph. Write notes of the main topics that you find and then do the same with the second text. Once you have done that it will be much easier to compare the ideas of the 2 texts from your notes.