You can add a hidden input (of the controlling field) in your second page

<apex:inputField style="display:none;" value="{!Product_Brief__c.System_Size_SUB_2013__c}"/> <apex:inputField value="{!Product_Brief__c.SUF_BPCs__c}"/> 

You can add a hidden input (of the controlling field) in your second page

<apex:inputField style="display:none;" value="{!Product_Brief__c.System_Size_SUB_2013__c}"/> <apex:inputField value="{!Product_Brief__c.SUF_BPCs__c}"/> 

You can pass parameter between pages like here

in your case, it will be something like:

public Pagereference page2() { PageReference pageRef = Page.page2; pageRef.getParameters().put('System_Size_SUB_2013',Product_Brief__c.System_Size_SUB_2013__c); return PageRef } 

and in the controller of page2 just put the value of the parameter in your constructor for example

public class page2Controller { Product_Brief__c.SUF_BPCs__c = System.currentPagereference().getParameters().get('System_Size_SUB_2013'); }