I don't remember, that somebody has said, that you must focus on one. And there is also no module for using both.
You could develop block and dodge, but it wouldn't be good. You would start to distribute your points to wide. If you would use all the points on only one defense ability, this ability would be better, than if you use it on two. And if you must use your "block" ability for "dodging": If your block is high enough, this -60 malus wouldn't be this bad.
For example, someone with inniate class boni an characteristic boni:
Attack 50, Dodge 50, Block 50 vs. Attack 75, Block 75 (or Attack 50, Block 100).
This 25 to 50 points is a huge difference AND it doesn't happend this often, that you must change your defense style, so the 75-60 = 15 for dodge, would be happening this often.