IP HELPER forwards DHCP REQUESTS. The workstation sends DHCP request when it HAS NO IP ADDRESS. It means that you cannot filter IP Helper requests based on IP addresses.
If you want to have a different DHCP servers for different PCs, you may install two or more DHCP servers (physical servers), configure certain MAC addresses to get a specific IP addresses (static DHCP assignment), and configure multiple "ip helper" addresses on your router. In this case your DHCP requests will be forwarded to ALL DHCP servers, but the only DHCP server with specific configuration (assignment) will reply to certain requests.
Mike CCNP, CCDP, CCSP, Cisco Voice, MCSE W2K, MCSE+I, Security+, etc. CCIE R&S (in progress), CCIE Voice (in progress)
ip helper does a whole heck of a lot more than just dhcp. Helper by default forwards 8 different UDP protocols, you can also customize what it sends. To answer the OP's question, no, you cannot tell one device to go one way, another to go the other way via the helper command. If you know that there is one specific device that needs specific information either hard code it or create another vlan for it and send that vlan where you want it to go.