Dear Stephanus,
This is possible through the standard functionality by maintain the special procurement keys 70, 80 in the BOM, if you want better clarity please go through the below path..
http://cwwongsap.blogspot.in/search/label/~%20PP%20Cross%20Plant%20Production
same way through STO process also you can proceed i.e. if you give demand at 1000 plant (Receiving plant) requirements will flow to the supplying plants (1100) as well as 1200 plant..
for maintain this you have to define the Quota arrangement in MEQ1 same way maintain the quota usage, lot size with quota in receiving plant material master then run the MRP system will generate the proposals based on your quota
Regards,
Venkatesh