I concur with the 3:2. Somewhere between 1:1 and 3:2.
Your attacking army needs enough defense to be able to survive the battle and, with 2 battle encounters, it needs to have enough units survive the first battle to win the second. Remember, the game opponents have their bonus equally split between attack and defense. If your attack and defense % have too big of split you can find yourself regularly loosing fights even when you vastly outpower the computer.
Let me give you an example to illustrate the point. (note: unit abilities, terrain, etc make the matter much more complex but I'll ignore them for this example). Let's say you decide to go all attack and have 3,000% attack and 300% defense. Lets say the computer opponent has 1,000% attack and 1,000% defense. You have a total combat bonus of 3,300% and your opponent has a total of 2,000%...you clearly have the more powerful army. Each unit does a range of damage from min to max. The game compares your attack% (3,000%) to your opponent's defense% (1,000%)...congrats! you have 3x as much and the distribution of damage between min and max will reflect that 3x advantage. Your opponent has an attack of 1,000% and you had a defense of 300%...so congrats to him ! he has a 3.3x advantage. His distribution of damage with a 3.3x advantage will be greater than your 3x ...you better hope your unit mix, terrain advantages and pure luck are on your side or you will loose even though you have a much stronger army !!!
Granted, the preceding paragraph is an extreme example. But, it illustrates the problem you can have if you let your defense lag too much. I agree that attack is more important than defense. You want a higher chance to do max damage, and when combined with the crit chance of an AO, you can get one shot kills. Dead enemy units can't hit back. So my advice, like the previous posters, is to keep it between 1:1 and 3:2.