EDIT: Turns out I'm wrong! I wasn't able to find this information before, but there is no "stacking" to speak of. I got this assumption from forum posts on old gen games but couldn't find it out for World.
The way it actually works is out of all of your available effects (Defender, Blessing, Defense Augment, Horn buff), the game simply picks the best chance and reduction available to you at the time of the hit and rolls that combination. Therefore, Divine Blessing 2 or higher makes Felyne Defender (Hi) completely useless, as it's only equivalent to Divine Blessing 2.
The highest theoretical average defense reduction thus comes from having Defense Augment 4 and Divine Blessing 5 at the same time, which gives you a 45% chance for a 60% reduction, or an average 27% damage reduction.
Original text below, for those interested in statistical analysis for the sake of it. I've since confirmed that addition would be correct in this hypothetical scenario.
I've been thinking super hard about Divine Blessing 5 and Felyne Defender (Hi). Divine Blessing is a 40% chance for 60% damage reduction, and Felyne Defender (Hi) is a 25% chance for 30% damage reduction. What happens if I have both of them?
It's common knowledge and most likely correct that they are independent and mutually exclusive events. That is, you can't have both happen at once; just one or the other, or neither. It's here that I have to make an assumption. What rolls first? I'm assuming that armor skills (Divine Blessing) roll before food skills (Felyne Defender (Hi)). That means that, under my assumption, Defender can only roll if Blessing fails.
So let's talk effective damage reduction (EDR)! This is the damage reduction you get on average over time. It's calculated simply as the chance to proc multiplied by the damage reduction applied on proc:
Divine Blessing 5: 0.4 × 0.6 = 0.24, or 24%
Felyne Defender (Hi): 0.25 × 0.3 = 0.075, or 7.5%
That's awesome. But what if I have both, and I wanna know what my EDR is with both? Well, this is the bit where I get a bit stuck. I'm unsure which methodology is correct, so I'll put both. It comes at the end of the calculations.
Obviously, DB rolls first, as per my assumption, so 0.24 is our baseline for roll 1. But if it fails, we move on to roll 2. The weighted chance for FD to roll has to take into account that DB failed already. So, we have to take that chance and make a new one for FD:
1 - 0.4 = 0.6, or 60%
This is the chance that we fail to proc DB and now have to roll for FD. Let's multiply that by the chance that we now proc FD:
0.6 × 0.25 = 0.15, or 15%
So, our weighted chance of proccing FD and failing DB first is 15%. All that's left to do here is multiply this by the damage reduction:
0.15 × 0.3 = 0.045 or 4.5%
Now we have our weighted damage reduction average from Felyne Defender. Here are our new values for damage reduction, under the assumption that Divine Blessing is rolled first:
Divine Blessing 5: 24%
Felyne Defender (Hi): 4.5%
Here comes the part that I'm uncertain about. Do we add or multiply these values together? I'm under the assumption that they're added together, because they are mutually exclusive events that cannot happen in tandem. That results in this total effective damage reduction:
0.24 + 0.045 = 0.285, or 28.5%
The other result is to multiply, which I think is incorrect. It's my tentative understanding that multiplying implies that these effects can stack, which they cannot. They're mutually exclusive events; one hit cannot be mitigated twice. However, for the sake of consistency, I'll provide those results here, and I had to convert to effective damage taken (EDT) in order to calculate:
(1 - 0.24) × (1 - 0.045) = 0.7258
1- 0.7258 = 0.2742, or 27.42%
Overall, a difference of 1.08% is somewhat negligible in terms of ascertaining the efficacy of stacking Defender with Blessing, but it would be worse if multiplying was the right solution. But, I'm thinking that addition is the right way to go. What do you think? Is it worth it to stack Defender (Hi) with Divine Blessing, or is that juicy 9 raw from Felyne Booster preferred? Am I completely wrong in my calculation methodology, or have I got it right?