MUME

Are they added as defense or are there two rolls to pass first one then the other?

Good question! I'm not super sure.

I do know that my def warrior can get over 100% pb, and still get hit, so it's clearly an opposed roll. Pehraps there are two rolls, but each opposed separately. Hopefully someone who knows more can chime in..

Your defensive warrior can get hit because defence is somewhat random these days. Anyone can get hit, no matter the defence. So that alone doesn't yet reveal the mechanism behind this. I'd be a bit surprised, though, if it really were two separate rolls... kind of goes against experience... I think?

Only someone with access to C code could answer this accurately. Others can only guess. I've always believed it was actually 2 separate rolls, first your DB + random factor is rolled (DB values are probably actually percents here) with levels (mostly your) likely affecting this as well, then your splitted (by number of enemies attacking you) PB is rolled against current attacker's OB again with a random factor (OB/PB can be above 100 so it is obviously not percents). Levels (mostly your) also probably affect effectivness of parry splitting. If the result is that you are hit, damage is calculated using many factors including all the mentioned above. I said I've always believed so, but nowadays I don't really know as that algorithm has been probably changed several times during the years (at least some of the randomization code was added, so it's possible that it was completely revamped).

I agree with elestir it is quite unclear how defence work (or better doesn't work) these days.
As basic aproach for newcomer i use following formula:

chance to get hit is

reallevel+random(0-100)+OB **VS** reallevel+random(0-100)+dodge+parry/attackers

Higher number wins.

I believe there is some damage bonus if one number is much higher than another and i believe this formula is miles away from what it really is, but it HELPS newbies to give impresion of what PB OB and DB is.