To most everyone's surprise, it appears that Ingus' TM Letters & Arms stacks on Global. As this TM competes directly with Shantotto's TM (MAG 30%) I wanted to compare how many Ingus TMs I would need to make them worth using over Shantotto's (since I already have four Shantotto TMs... don't judge me) but figured I would post this in case anyone else is interested in grinding out mage TMs and to decide which are better for you (since many of us aren't hurting for Shantottos) and if it's therefore worth pulling for multiple Ingus, and if so, how many.

I've used Exdeath as the example (both at 5★ and 6★), as he'll be our top mage "soon" and can equip swords. Terra has very similar base MAG to 5★ Exdeath (Terra at 109 -- Exdeath at 112) and the conclusions are the same between the two of them.

The charts below are only calculating the MAG gained from ability TMs and the mage's main hand weapon. Other MAG (base stats, passives, other equipment, esper, etc) do not make a difference.

As you can see above, for 5★ Exdeath, you need 2 more or more L&A to make it worthwhile over MAG 30% if your best rod is the Lilith Rod, you need 3 or more L&A to make it worthwhile over MAG 30% if your best rod is the Wizard Rod, and you need 4 or more L&A if your best rod is (or will be) the Omnirod.

For 6★ Exdeath, you need 2 or more L&A to make it better than MAG 30% unless you also have Omnirod, and then you need 3 L&A.

Since people are asking about DW, it's the same breakdowns as the above chart, but to clarify here's a breakdown when using DW based off your SECOND BEST Rod vs Enhancer. Your best rod would be in your other hand and is therefore not a variable:

In this situation you only need 2x L&A if your second best rod is the Wizard Rod or worse, but 3x L&A if you happen to have an Omnirod.

If it's a character that cannot use rods (like Ramza) then obviously L&A beats MAG 30% in any configuration -- for mages that cannot use swords (like Kefka) MAG 30% will beat L&A in any configuration.

TL;DR: If you plan to stack Ingus TMs on a mage that can use rods or swords you should probably plan to get at least 3 (per character) to make them worthwhile long term.