You use treat them as separate Tests. If you put one die in attack, and roll against Ob 1, you can get a Difficult Test. If you log a Challenging Test, it’s because you either failed or used Artha to make up the difference; good for you!
Making a game of Advancement is a perfectly sporting thing to do in Burning Wheel. I expect part of the reason for this set-up is to facilitate that gaming.
If you want to hinder yourself for Test in Fight, that’s great! And risky! Even if you’re shorting yourself on the attack, leaving your opponent unharmed to hurt you next Action is a risk.
One of my favorite ways to game advancement is in Duel of Wits (usually against another PC). If they’re using Beginner’s Luck, I’ll script a Beginner’s Luck Rebuttal with Rhetoric. Then either short Attack to one die, but pump in Persona to succeed on a Difficult, or short Defense to one die, but pump in Persona to try to succeed on a Challenging!