Is it permissible to change the intention of buying two copies of the Qur’an with each salary and transfer to buying something else, and is it obligatory to place them in the same mosque, or is it permissible to distribute them to different mosques?

As long as you promised Allah with your heart and did not utter it with your tongue, nothing is binding upon you. If you wish to proceed with what you intended, that is good, but you may also change what you intended. A mere intention and self-talk do not entail anything binding, as the Prophet, peace and blessings be upon him, said: "Indeed, Allah has pardoned for my Ummah whatever crosses their minds, as long as they do not act upon it or speak of it."