Doubt in selection screen

Hi all.

I have two input fields one for material other plant.

I have done select option for material (because i need to give more then one material number)

I have parameter for plant .

I need to validate plant corresponding to material number.I have done it for single material input.

How can i check for one more material number in select option.

eg:

i give 3 material number by selection option:1001,1002,2001

plant:001

material number 1001 and 1002 belong to plant 001 but material number 2001 belong to other plant.

how can validated.?

To be Reward all helpfull answers/

Jay