I have one cluster EKS cluster already running and I just created a second one. I'd like to use the same ELB that is being used for the first in the second's ingress controller. Is is possible?
The short version is yes, the long version is "but you for sure do not want to".
If one looks at the way traffic arrives from the wild Internet into your cluster:
{ Internet } --> ELB --> Node --> NodePort --> Service
it's more helpful to see why the answer is "yes" but also why the answer is "but don't do that" since in your hypothetical situation you'd have
{ Internet } --> ELB --> Node --> NodePort --> Service
\
--> cluster-2-Node --> cluster-2-NodePort --> cluster-2-Service
meaning that the ELB would round-robin requests across both clusters because it wouldn't have any way to know which machines to send the "real" traffic to. Now, if your clusters happen to be perfect mirrors of one another, lucky you, it'll probably work for a while1 but it's for sure swimming upstream of the way kubernetes and ELBs want to work. The less "server-fault-question-asking" way of doing that is to make a CNAME for both ELBs and then let DNS round-robin the traffic
fn 1: the "for a while" part is that the association between the ELB and the instances to which it is registered are kept in sync by kube-controller-manager and thus when its reconciliation loop runs and it finds "foreign" Nodes attached to "its" ELB, it'll evict them
