The CLI must be used to configure multiple DHCP pools on one interface as this cannot be done using the GUI.
config system dhcp server
edit 1 // Pool 1 is in the 10.129 network
set auto-configuration disable
set conflicted-ip-timeout 1800
set default-gateway 10.129.0.113
set dns-service specify
set domain ”
set enable enable
set interface “internal”
config ip-range
edit 1
set end-ip 10.129.0.20
set start-ip 10.129.0.10
next
end
set lease-time 604800
set netmask 255.255.252.0
set next-server 0.0.0.0
set option1 0
set option2 0
set option3 0
set server-type regular
set vci-match disable
set wins-server1 0.0.0.0
set wins-server2 0.0.0.0
set dns-server1 0.0.0.0
set dns-server2 0.0.0.0
set dns-server3 0.0.0.0
next
edit 1
https://travelingpacket.com/2013/11/27/fortigate-dhcp-server-via-cli-and-adding-dhcp-options/
https://kb.fortinet.com/kb/documentLink.do?externalID=FD33842