Is Stacking Day/weather Gear Always Best?

For example:

Twilight cape/obi/sorcerer's tonban VERSUS Taubran cape/othila sash/akasha pants.

Is day/weather gear ever excessive? Is there a magical combination of the two types of gear that yields the best results?

Should I only use day/weather gear when BOTH conditions are met (i.e. fire weather AND fire day)?

Short answer, yes always use day/weather gear.
There's a cap but you won't hit it without double weather.

absolutely. potency > matk > int > macc

The Day/Weather step of the magic damage formula caps at 40%, using the following static numbers for each variable:
10% single weather
25% double weather
10% matching day
10% Iridescence proc
5% Relic legs
5% Twilight cape
3% Zodiac ring

With that said, it is possible to reach the cap. If you have the weather, it is more beneficial to stack weather gear and obi, since this step is calculated before the MAB step.

Example:
Say Thunder 5 deals 1500 damage not factoring in weather or MAB, and your gear, not counting back/waist/legs has 50 MAB, and single weather.

Forcing weather would be
1500
*1.2 (10% weather, 5% legs, 5% back)
----------
1800

1800
*1.5 (MAB)
========
2700

Vs

1500
*1.65 (Base gear MAB+ akasha/taubran/othilia)
======
2475

You can hit it without double weather, but you need to be using Chatoyant Staff But doing this reduces your overall damage.

Thanks so much everyone! Slowly making my BLM better...