Hello Willian
This is the standard process of production in alternative plant.
I suggest you to consider the special procurement "40 - Stock transfer". MRP will then generate a stock transfer from plant 1800 to plant 1000 and a planned order will be created to cover the stock transfer release on plant 1800.
You will convert the planned order to production order and the goods receipt will be posted on plant 1800. After that, you will transfer your material from plant 1800 to 1000, based on the stock transfer.
BR
Caetano