Only few tillandsias need a lot of direct sunlight, others are fine even with indirect light. But there is one much more important rule when growing them - they must get dry fast after watering (within 4 hours recommended). Your problem is typical example of overwatering - plants didn't get dry fast enough. After watering turn them upside down or at least lay/mount them horizontally, so leftover water can flow out of the plant center. Add some air flow. Open the window over summer or use a tiny ventilator over winter. Try to move them to a warmer place if the windowsill is too cold.

About fertilizing - bromeliad/orchid fertilizer is fine. Fertilize only over summer season, once per 3-4 weeks. Use at least 10x lower concentration than is recommended for orchids or big bromeliads. For example I use only 10 drops of fertilizer for 1 liter of water.

Your plants might be t. ionantha (small) and t. brachycaulos (big green), they are most commonly sold species in shops. T. ionantha needs more sun and careful watering over winter. T. brachycaulos is very tolerant. It can have less sunlight and more water, dislikes high summer temperatures.

If you can hang them up, using fishing line or non-copper wire or yarn or cotton shoe laces or maybe dental floss, that will allow more airflow. I have a tack in my bedroom ceiling (nothing more, I rent this apartment so no building projects), near a south-facing window, and from it I have some twine hanging and at about my shoulder height I tied a loop and I hang from that line Spanish Moss. I should put up more tacks and hang more twine for other light plants (but not Xerographica, for example, too heavy). Do an internet search for "hanging Tillandsias" and you'll see plenty of examples to give you inspiration as long as the plant is not enclosed and cut off from air.

Thank you both, this has helped a lot! I think I might be able to keep these guys healthy and gorgeous better now.