The way to achieve this is to configure each router with a DHCP pool that is half of the pool that you intend to use. Each router will offer addresses out of its pool. With both routers active when a client sends a DHCP request it will receive a response from both routers and will choose which offer to accept. You will probably wind up with some clients using addresses from router A and some clients using addresses from router B. With one router down the other router will still offer addresses.
The biggest drawback of DHCP is that there is no synchornization between different DHCP Servers, hence if you want to deploy more than 1 server, its always a challange.
Different OS vendors address it differently, by the way, if you have Windows ADS, then the best way is to incorporate it through the DC or ADCs.
However if you are not interested using any OS, then , the best strategy would be to divide the Scope / Pool between 2 servers (in your case Routers) and let both the routers assign the IP Addresses independantly, so that in case of one Router's failure the second Router would be able to take care of the situation.
A second but less preferred idea would be to assign a longer lease period e.g. 15 days lease period, during which the existing clients won't run out of options and if there are any new clients, you can assign them manually.
There is not a special command. You just put half the range into the pool on one router and the other half of the range into the pool of the other router. To do it your configs might look something like this (assume that you want 10.6.2.0/24 to be the pool of addresses for DHCP):
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