Ah. So missing "fields".
If the overall goal is to blend the term stats from 2 fields in 2 indices the answer is "no".
Generally, significant terms will work best on a single index and single shard since all of the stats are available in one place. If you're trying to use it to spot low-frequency terms (e.g. something that only occurs twice) in a distributed system that makes life hard because every single-occurrence term on a local shard (of which there are typically many) suddenly becomes a candidate for global consideration.