Kurotowa said:
Although I wasn't responding to you, as I look back at your post I think I did cover your point. Just because unnamed bonuses can stack does not mean they do stack.
You might want to check the rules on unnamed bonuses again.

(PHB, page 171-172,
Combining Magical Effects,
Stacking Effects,
Different Bonus Names, last sentence: "A bonus that isn't named ... stacks with any bonus."
As the spells point out they are creating a disguise, not enhancing it. If you put an illusion on top of a transformation, all that is seen is the illusion.
Not actually true. Or at least, nothing in the rules actually state that. As has been pointed out, the spells can be used affect different aspects of the disguise (
disguise self for equipment and clothes,
Alter Self for physical form and appearance. You don't have to have the illusion "covering" your entire body, you can use it for certain parts, and the transformation for other parts.
If the illusion had to be "on top" of everything and "cover" your entire body, the
only bonus you would be able to use would be your skill and the spell bonus. You wouldn't be subject to any of the bonuses or penalties listed to the Disguise Check listed on page 73 of the PHB.
With Disuise Self a male elf, half-elf, or half-orc would all gain the same +10 to disuise themselves as a human female.
Not exactly. There's an additional penalty for disguising your race, something the elf (and possibley half-elf) wouldn't have to do.
So why would it matter would it matter which of those bodies you take on with Alter Self or as a Changeling before you use the Disguise Self?
Because you can then use the
Disguise Self spell to focus on other aspects of the disguise, like your equipment. Things not handled by
Alter Self or the Changeling ability. But mainly because the rules are pretty clear about unnamed bonuses stacking. Especially when it's not the same effect generating the bonus.
Because Alter Self and Disguise Self have the same degree of mastery with a disguise (+10), we must infer that neither is better than the other at fooling others as to who and what you are, even if the Disguise Self does things that Alter Self does not (illusionary clothing and equipment).
No, we don't have to infer that at all. I don't think the amount of the bonus has anything to do with stacking.
Example: Skill Focus give me a +3 bonus on a skill. Having a Familiar can give me a +3 bonus on a skill as well. Are you claiming that since it's the same total bonus it wouldn't stack, even though it's from two different sources that grant the bonus in different ways?
The two spells are different effects, and grant an unnamed bonus. By the rules, they absolutely stack. (If they were both illusion or both transmutation you could make a case for "same effect more than once in different strengths", but that's not what is happening here. This is why I don't think
Alter Self would stack with
Polymorph X.)
If both do the same thing (create a disguise) with the same result (+10), how can they be combined?
Because they do it in different ways, and the bonus is unnamed. The amount of the bonus is irrelevent, only the type of bonus and the way that bonus is achieved.