Question: Why can’t there be more than one Buddha in one Pure Land?
Answer: Each Pure Land karmically arises as a result of one’s spiritual purification, as expressed by physical purification of one’s physical environment too. As such, there is no further room for this purification to be expressed in an already pure land. This is why there is only one Buddha in one Pure Land.
Also, the manifestation of more Pure Lands is an expression of great compassion, to reach out to benefit more beings. Think of this as it being more convenient for more to have more ‘convenient stores’ to access, according to one’s karmic affinity. (However, Buddhas-to-be, who are greatest of Bodhisattvas ready for Buddhahood can replace Buddhas in their Pure Lands.)