Much like in Eve Online, the only way to assuredly nyx dual-tanking is to make it simply not worth doing. And that does not mean making it so that dual-tanking is impossible or counter-intuitive through penalties, it simply means providing modules in both high and low slots are just better to run.

Look at Armor Tanking in Eve Online and it opens up your mid-slots for a plethora of EWAR opportunities, like tackle and mobility.
Look at Shield Tanking and it opens up for damage amplification, tracking enhancement, mobility, and cargo space.

Players can dual-tank in Eve but they choose not to because the options available provide for much more diverse and generally just better fittings than you'd get if you tried to brick down. We need to consider that for Dust 514, I feel. Carrot, not the stick.

Dual tanking can be reduced by making non-tank related mods more relatively effective compared to tanking mods, either by buffing other mods or nerfing tanking mods. This is effectively the carrot approach Aeon describes.

Other than non-tanking mods, regen mods are very important for reducing dual tanking. This is because they produce synergy between modules of the same tanking type, encouraging you to focus on one type. Shield regulators and shield delay are crucial as they are shield modules that compete with armour modules. They have to be more worthwhile than armour on a shield suit. This is why I caution against reducing base delays too much as it diminishes the effectiveness and value of regulators. If they delays are deemed too long, better to buff the regulators to give people the tools to reduce them if they give up on dual tanking.

You bickering trolls have derailed this discussion with your petty personal grievances. Cease and desist.