From your description you need to do the following:
Create two separate Origin Pools in the load balancer.
Put the Primary site IP(s) in the first pool, and the Secondary site IP(s) in the second pool.
Attach a health check to all servers in both pools.
Set traffic steering for the load balancer to ‘off’. This makes CF choose the correct pool in failover order, top to bottom.
With only one pool, the best you can do is set the weightings to prefer one, but the secondary site will still get some traffic even if the primary is healthy. With two pools you can set different steering methods to balance the traffic (or failover in this case.)
Traffic Steering is one of the settings on a Cloudflare load balancer.
You can set it to direct users to the closest origin, or to direct by geographic region. In your case, the setting should be “off”, which makes the load balancer failover between the pools in the order you define (primary, then secondary)