Is buying necessities in installments for people after collecting money from them through a subscription and shares system—given that these necessities are only available to you upon request (recommendation)—considered part of "two sales in one sale," and is it forbidden?

If the transaction involves people asking to buy something and promising to purchase it from you, and for you to make a profit from it, this is known as Murabaha for the one who orders the purchase (Murabaha li al-amir bi al-shira'), and it is permissible with its conditions, and it is permissible to be on installments. This does not fall under the category of "two sales in one sale." However, if you sell them things before you own them, hoping to acquire them, then this is selling what one does not possess, which is invalid due to the Prophet's (peace and blessings be upon him) saying: "It is not permissible to sell what you do not possess."