"witch1 already doing DHCP Relay.. yes?"
Yes.
"Why cascade Relay on switch2.. how about a trunk link between the Switch2 to switch1.
For instance:
PC DHCP/BOOTP => SW1 <Trunk> SW2 => DHCP SERVER"
Such a network design, DHCP server is outside location. New switch need for connect new users. Of course between switch1 and switch2 trunk link.