ARP operates on the interface and responds to every request coming from either the primary or secondary subnet. When multiple subnets are configured on a VLAN and an ARP request is generated by the switch over that VLAN, the source IP address of the ARP request must be a local IP address of the subnet to which the destination IP address (which is being ARPed) belongs.
For example, if a switch multinets the subnets 10.0.0.0/24 and 20.0.0.0/24 (with VLAN IP addresses of 10.0.0.1 and 20.0.0.1), and generates an ARP request for the IP address 10.0.0.2, then the source IP address in the ARP packet is set to 10.0.0.1 and not to 20.0.0.1.