I'm fairly certain that when the router forwards the DHCP request it includes only the primary network configured on the interface. I don't believe there's any way to have it use the secondary network.
Edit: I did some Googling and determined what others have already pointed out, there's a feature called smart-relay that can use the secondary network, but only after the primary times out three times.
The relevant quote from Understanding and Troubleshooting DHCP in Catalyst Switch or Enterprise Networks - Cisco Systems
How to make DHCP Work on Secondary IP Segments
By default, DHCP has a limitation in that the reply packets are sent
only if the request is received from the interface configured with the
primary IP address. DHCP traffic uses the broadcast address. When the
DHCP request is received by the router interface, it forwards it to
the DHCP server (when IP helper-address is configured) with a source
address of the primary IP configured on the interface to let the DHCP
server know which IP pool it must use (for the client) in the DHCP
There is no way for the router to know if the DHCP broadcast request
comes from a device that is on the secondary IP network configured on
the interface. As a workaround, sub-interface configuration (provided
that the device connected to the router supports dot1q tagging) to
separate the two subnets can be configured, so both of them get their
correspondent IP addresses properly.
If the secondary address is the preferred way, there is another
workaround, which is to enable the global configuration command ip
dhcp smart-relay . This has a limitation in that it only uses the
secondary IP to relay the DHCP request if there is no response from
the DHCP server after three consecutive requests for the primary