Evangelist and Orphanage Founder, Geroge Mueller:
George Müller, the geat evangelist and coordinator of orphanages said this about reading both the Old and New Testaments concurrently:
"It is of immense importance for the understanding of the Word of God, to read the Bible systematically, so that every day one reads a portion of the Old and a portion of the New Testament systematically." - George Müller
First, this method is important because it throws light upon the connection between the Old and New Testaments. A different method, where one habitually selects favorite chapters, will make it utterly impossible ever to understand much of the Scriptures.
Second, while we are in the body, we need a change, even in spiritual things; and this change the Lord has graciously provided in the great variety that is to be found in His Word.
Third, an orderly reading of the Word honors the glory of God. The leaving out of some chapters here and there is practically saying that certain portions are better than others, or that there are certain parts of revealed truth that are unprofitable or unnecessary.
Fourth, it may keep us, by the blessing of God, from erroneous views, since in reading regularly through the Scriptures, we are led to see the meaning of the whole. We are also kept from placing too much stress upon certain favorite views.
Fifth, the Scriptures contain the whole revealed will of God; therefore, we ought to seek to read from time to time through the whole of that revealed will. I fear that many believers in our day have not read even once through the whole of the Scriptures; yet in a year, by reading about 3 chapters everyday, one can achieve it.