On a side note, I'm a little confused by the "-1 OB for each -1%". Does this mean if I spend 75% to make my attack, there's a -25 to my attack? And if I spend 50%, there's a -50? (But I could potentially make 2 attacks under normal rules?)
Yes. Any action has a requires Activity Percentage. Some actions can be performed over a range of percentages. However, if less than the maximum amount is use, then the character receives -1 to his OB for each 1% short of the maximum percentage, the character receives -1 to OB (or to other maneuvers performed).
So, if a character makes a 25% activity move and then a 75% activity attack, he receives a -25 to his OB.
As for taking a -50 to get a second attack, the core rules normally only allow a single attack roll per round (be it melee, missile, or spell). There are exceptions built into the system though for TWC, and so forth (as explained in Arms Law.
If you ask for a ruling, I would say no to 2 attacks at -50 each without using styles or twc or something along those lines that will allow for such.