Is it possible to merge multiple ingresses with the IBM Cloud Kubernetes Service?
Yes. You can have them on different namespaces you can read more about it here
You can also read about it on this Github issue
You can also have multiple ingress controllers for example with nginx ingress controller by specifying the --ingress-class option on the nginx ingress controller command line. More information here.
The IKS documentation lays out a few different scenarios for managing ingress. It sounds like you're looking to handle multiple apps, but it's not clear if you're doing this in one namespace or multiple namespaces. If multiple namespaces, it does require multiple ingress resources. If in one namespace, you have some more flexibility on how to structure your ingress files: https://console.bluemix.net/docs/containers/cs_ingress.html#multiple_namespaces