Creating an index from two others using logstash

Hello,
Being fairly new to the API, I am having a hard time trying to create an index containing some fields {a, b, c} from two other indexes containing some {a, b} and {b, c} fields respectively (there is a 'b' field once and only once in both indexes). Is it a job for es' pipeline ? Or is it a task logstash should be carrying out ?

Thanks in advance for any hint