How to add rules to Kubernetes NGINX Ingress controller from different yml definitions?
I was able to combine your two example files by first converting them to JSON. 

foo.json

{
  "apiVersion": "extensions/v1beta1",
  "kind": "Ingress",
  "metadata": {
    "name": "nginx-ingress"
  },
  "spec": {
    "tls": [
      {
        "hosts": [
          "foo.bar.com"
        ],
        "secretName": "tls-secret"
      }
    ],
    "rules": [
      {
        "host": "foo.bar.com",
        "http": {
          "paths": [
            {
              "path": "/",
              "backend": {
                "serviceName": "foo-web-svc",
                "servicePort": 80
              }
            },
            {
              "path": "/api",
              "backend": {
                "serviceName": "foo-rest-svc",
                "servicePort": 80
              }
            }
          ]
        }
      }
    ]
  }
}


baz.json

{
  "apiVersion": "extensions/v1beta1",
  "kind": "Ingress",
  "metadata": {
    "name": "nginx-ingress"
  },
  "spec": {
    "tls": [
      {
        "hosts": [
          "baz.bar.com"
        ],
        "secretName": "tls-secret"
      }
    ],
    "rules": [
      {
        "host": "baz.bar.com",
        "http": {
          "paths": [
            {
              "path": "/",
              "backend": {
                "serviceName": "baz-web-svc",
                "servicePort": 80
              }
            },
            {
              "path": "/api",
              "backend": {
                "serviceName": "baz-rest-svc",
                "servicePort": 80
              }
            }
          ]
        }
      }
    ]
  }
}


You can then combine the rules for each by using jq.

levi@La-Tower:~$ jq 'reduce inputs as $i (.; .spec.rules += $i.spec.rules)' foo.json baz.json
{
  "apiVersion": "extensions/v1beta1",
  "kind": "Ingress",
  "metadata": {
    "name": "nginx-ingress"
  },
  "spec": {
    "tls": [
      {
        "hosts": [
          "foo.bar.com"
        ],
        "secretName": "tls-secret"
      }
    ],
    "rules": [
      {
        "host": "foo.bar.com",
        "http": {
          "paths": [
            {
              "path": "/",
              "backend": {
                "serviceName": "foo-web-svc",
                "servicePort": 80
              }
            },
            {
              "path": "/api",
              "backend": {
                "serviceName": "foo-rest-svc",
                "servicePort": 80
              }
            }
          ]
        }
      },
      {
        "host": "baz.bar.com",
        "http": {
          "paths": [
            {
              "path": "/",
              "backend": {
                "serviceName": "baz-web-svc",
                "servicePort": 80
              }
            },
            {
              "path": "/api",
              "backend": {
                "serviceName": "baz-rest-svc",
                "servicePort": 80
              }
            }
          ]
        }
      }
    ]
  }
}


While yaml is more common JSON is perfectly valid for the Kubernetes API. All credit for the jq logic goes to @Jeff Mercado and his answer.