It is inappropriate to place immortals in Buddhist halls. Although it is not a problem to place statues of gods in Buddhist halls, there is no need to place statues of gods. Because in Buddhism, gods are all sentient beings in the six realms of reincarnation, but they are very high-level beings with long lifespans, great blessings, and are willing to help people. But they are not as vast and boundless as Buddhas and Bodhisattvas.
If you want to place it, put it in the same niche with a handful of deities and Buddha statues, but they should not be placed side by side, but the deity statues should be placed behind the Buddha statues.
There are some things that I don’t know whether to say or not. Master Jian Xinhui, a famous Maoshan Taoist master in Hong Kong, has said this problem. He does not advise us to enshrine Buddha and deity images in our homes. Because in many cases if it is not handled well, what is being worshiped may just be a ghost. Unless the Buddha statues and deity statues must undergo strict consecration, and the location of the altar must be suitable. But this raises a question. Buddha statues must be consecrated by monks, and divine statues must be consecrated by Taoists. Therefore, it is not legal to put them together. Moreover, many Taoist priests nowadays are not authentic Taoist priests. If the consecration is not done well, it is easy for the gods not to be invited but ghosts. This greatly violates our original intention. Many people worship statues to pray for wealth, promotion, health and longevity. However, ghosts are often used to accomplish this. Because it is difficult to pray to gods, ghosts are usually used to accomplish these tasks. Only masters with high moral standards can truly invite gods to come down. If you understand Cantonese, I can give you the recording of Master Jian Xinhui. He has already explained these things in a very comprehensive way. .hk/ is this website.
To give you another proof, you can look at the differences between Buddhist and Taoist consecrations online. There are many on Youku. Buddhist consecrations basically have auspicious signs, while Taoist consecrations rarely have auspicious signs. This greatly proves the vastness of Buddhism and the poor conduct of many Taoist priests today. I am not belittling Taoism, but the facts are before our eyes and we have to be vigilant. After all, Buddhism still has a great influence in China, so in comparison, Taoism seems to be much less elite.
So, it is enough to enshrine one Buddha statue, and there is no need to enshrine another god.