Need two Prometheus metric values in a single promql query

You can't do that with Prometheus, because its data model only supports one value per sample.

To store multiple values per sample, you need to use a time-series database that supports fields, such as InfluxDB.

You can use the or binary operator in Prometheus.

http_request_duration_seconds_bucket or 
http_request_duration_seconds_count

Here is a demo working example using instance as a common label for two different metrics

alertmanager_alerts_received_total or
alertmanager_alerts_invalid_total

Result:

alertmanager_alerts_received_total{instance="demo.robustperception.io:9093", job="alertmanager", status="firing", version="v1"}
0
alertmanager_alerts_received_total{instance="demo.robustperception.io:9093", job="alertmanager", status="firing", version="v2"}
635876
alertmanager_alerts_received_total{instance="demo.robustperception.io:9093", job="alertmanager", status="resolved", version="v1"}
0
alertmanager_alerts_received_total{instance="demo.robustperception.io:9093", job="alertmanager", status="resolved", version="v2"}
0
alertmanager_alerts_invalid_total{instance="demo.robustperception.io:9093", job="alertmanager", version="v1"}
0
alertmanager_alerts_invalid_total{instance="demo.robustperception.io:9093", job="alertmanager", version="v2"}