My Dendrobium Nobile Apollon has fungus ...
Is it possible that this fungus could infect other plants like my Hoyas?
Yes, the possibility is always there. Generally it is a good approach to isolate infected plants from the rest let it be insects or fungus.
You cannot really do harm with isolation given you have a place to put the plant away while giving it acceptable conditions. But if you keep them close/together the risk will be there and even if it is small, I personally do not think it is worth to take.