If you have two OS installed on the machine then there is no way that you can achieve this.
In the ePO server, only entry will be present where you have installed the agent.
If you install agent on both the OSes (XP and Win7) then it will create duplicate entry with different GUID in the ePO server.
In this case, I will suggest you to use disable MAC address search as per KB#:KB57987 if you have single NIC.
Even if you disable MAC address search in ePO server, you have to install the agent on both the OS (XP and Win7). Only thing, you will avoid is creating duplicate entry.