You can’t use the same infusion twice
The Infusing an Item part of the Infuse item feature (E:RftLW p. 57) includes
each of your infusions can be in only one object at a time.
so you can't use it on both your shield and armour.
You can't get around this by knowing the same infusion twice, as (E:RftLW p. 61)
Unless an infusion's description says otherwise, you
can't learn an infusion more than once.
And the only infusion with such an exception is Replicate Magic Item.
You can't use worn infused armor as a focus.
Your analysis of the "Tools Required" section is correct. You must have an infused item in your hand if you want to use it as your arcane focus. If you're wearing infused armor, you're not holding it in your hand, so you can't use it as your focus.
Even if the infused armor includes gauntlets, you won't have all of the infused item in your hand. You'll only be holding part of it, so a strict reading of the rules will conclude that it's not eligible to be a focus.
Your DM may make an exception for flavor.
One of the great things about being an artificer is figuring out how to re-flavor your spells, describing how exactly they manifest via your mastery of magical substances and devices. (See "The Magic of Artifice" sidebar on page 11 of Tasha's Cauldron of Everything for reference.)
Because of this class emphasis on creative re-skinning, your DM may be open to an especially fun idea about how to cast one or more spells through your gauntlet. Discuss it with them outside your play session. They may choose to allow it, or or the two of you may come up with an alternative that you'll both find acceptable.
An Armorer's worn arcane armor can be used as a focus.
The Artificer's Armorer subclass can use worn armor as a spellcasting focus, but it can't be just any infused armor; it must be the arcane armor granted by the subclass's 3rd-level feature. While wearing it:
You can use the arcane armor as a spellcasting focus for your artificer spells.
Note that this armor can hold an infusion just like any other armor. Furthermore, at 9th level the Armorer gains the Armor Modifications feature which allows the arcane armor to hold multiple infusions. However, these infusions have no bearing on the arcane armor's ability to be used as a spellcasting focus when worn.
Related: Can an Armorer Artificer wearing Guardian-model Arcane Armor cast a somatic spell without holding tools or having the Warcaster feat?
The cleric has a similar, though rarely used, ability.
It's interesting to note that, if a cleric's holy symbol is emblazoned on their armor, they too can use their worn armor as a spellcasting focus.
This almost never comes up, though, because most clerics use a shield with their holy symbol on it as their focus instead.
Related: Do you need a free hand for your holy symbol if you're wearing it visibly?
Best Answer
Armor changes how AC is calculated. If you have multiple ways to calculate AC, you choose which one you want to use. If you have Unarmored Defence, Mage Armor and Barkskin, you only benefit from one of those.
So, even if you manage to don both armors, you will have to choose which one to actually use.