Kashif Minhas's questions

I'm configuring HSRP having scenario described in figure below. One gateway is active for one network and standby for other network and vice versa for other gateway. DHCP is configured on both Gateways and clients get automatic IPs. My question is if one Gateway (Switch A) is down other Gateway (Switch B) automatically becomes active for network for whome it is standby. Which DHCP option we need to configure on both Gateways DHCPs so that when Switch B is active and Switch a is down, and another client plugs in to network for which Switch A is active how come Switch B knows which IPs are allready assigned by Switch A and Switch B assign IPs to new clients other than IPs previously assigned by Switch A.