Small business server does, by default, only allow one DHCP-server on the network. This can be changed with a registry key.
On your main DHCP-server, open Registry Editor, navigate to
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Dhcpserver\Parameters
On the Edit menu, click New->DWORD Value, and then add the following
registry value:
Value name: DisableRogueDetection
Data type: REG_DWORD
Radix: Binary
Value data: (Hexadecimal) 1, which will be saved as 0x00000001
Then, please restart the server and test issue again.