Hi Sun,
I dont see any challege new creating a new plant for planning purpose even though its geographically located in almost same facility. All you need to do it to use the best practice functionality that suits the requirement. You can use special procurement key for production in alternative plant bu setting up plant as planning and other as production. The details are mentioned below:
Multi Plant Planning - Part 1 (Production in Alternate Plant)
But since your plants are very close there is no point having STO process, you can also look for SPK withdrawal form alternative plant. Try exploring the possibility.
Regards,
Rohit