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Would you be able to send a picture? I am not quite certain it is a fungus.

But, until you can send a picture and we can positively identify it, it would be a good idea to treat the plant with a fungicide. If it is a fungus, it will help stop it. And if it is not, the fungicide will not hurt the plant.

A Staghorn Fern needs to be mounted or grown in a basket.

This article will help you.

https://www.gardeningknowhow.com/ornamental/foliage/staghorn-fern/staghorn-fern-information.htm

Staghorn ferns need a lot of light, so it is preferable to keep your fern near a window in your house rather than relying only on grow lights. If the basement is your only option, the more light you can give it, the better. CFL lights or LED lights are good options:

https://www.gardeningknowhow.com/houseplants/hpgen/what-are-grow-lights.htm
https://www.gardeningknowhow.com/houseplants/hpgen/fluorescent-lighting-for-indoor-gardening.htm

As for timing, you could light them for 12 hours on and 12 hours off, or any other combination, as long as they get at least 8 hours of light and 6 hours of darkness.

It is normal for the flat leaves to turn brown and papery with age.
It is best to water as needed, not on a schedule.
This article will help refresh you on the care requirements.

https://www.gardeningknowhow.com/ornamental/foliage/staghorn-fern/staghorn-fern-information.htm

A Staghorn fern would not be symbiotic. The combination that you mention seeing in the garden center was likely created for visual interest, but the Maidenhair Fern has a completely different growing condition than the Staghorn. They would not thrive together for any length of time.
There has been a big popularity in the art of Kokedama moss ball plantings. This pairing seems to me to be a representation of this technique.
Here are some links with more information.

https://www.gardeningknowhow.com/ornamental/foliage/staghorn-fern/staghorn-fern-information.htm
https://www.gardeningknowhow.com/ornamental/foliage/maidenhair-fern/maidenhair-fern-care.htm
https://www.gardeningknowhow.com/garden-how-to/projects/making-kokedama-moss-balls.htm

Your fern may be responding this way because of the change from outdoor to indoor conditions. However, I think it is normal for them to lose a few fronds once in a while.

Staghorn ferns can be sensitive to the chlorine in tap water, especially if they've been watered by the rain outdoors. Try adding filtered water or tap water that's been left out for 24 hours to let the chlorine evaporate. This plant also prefers a moderately humid environment, so if the air is too dry, try misting it often or adding a humidifier. Here is more information:

http://edis.ifas.ufl.edu/mg015