Dynamically gray-out (disable) element/card

Hello and happy new year to everyone.
I have two picture entity cards acting as triggers for an automation that turns my water heater on, each for different time periods. The ‘shower’ option for 10 minutes and the ‘bath’ option for 20.

Screenshot_20200101-183330_Home Assistant
These two should be mutually exclusive. Ideally when I turn one on, the other should be grayed out (its tap_action disabled). Is this possible somehow?

With the ‘shower’ state on, I shouldn’t be able to interact with the bath card, they should never both be active at the same time:
Screenshot_20200101-183342_Home Assistant

I don’t want to hide the second card completely with a conditional. I believe custom components that ‘lock’ elements or cards don’t work dynamically either.

I’ve only found this semi-relevant reddit discussion, where someone mentions groups as a solution. But groups would only work(?) for single entities and not whole lovelace cards, correct?

This is my code, I’m toggling input_booleans and the automation catches the state change. I’ve tried using [state of shower = ‘on’] as a condition to block the bath automation but it’s not working.

type: horizontal-stack
  - type: picture-entity
    entity: input_boolean.water_shower_now
    name: Shower
      action: toggle
  - type: picture-entity
    entity: input_boolean.water_bath_now
    name: Bath
      action: toggle

The custom button card allows you to dynamically change locking. You can also style the card to look disabled if you don’t like the lock.