The approach you proposed should work as good/bad as your original single query in theory. I would be surprised if this approach works better than your original single query, but if so, please let me know.
The optimal approach depends on your data. Since each of your indexes only covers a single index, there are two strategies in general:
1. Leverage one index to return results for one condition, and do in-memory filtering for the other condition. Your original query does this. This is useful in some scenarios (e.g. one index returns many more results than another index).
2. Leverage both indexes and do in-memory intersection to return results that satisfy both conditions. This might be something you want, and it is useful in some scenarios (e.g. both indices return roughly the same results and there is not much overlapping between two sets of results). If this is what you want, try rewriting your query as g.V().has("indexed-prop1", "value1").or(__.has("indexed-prop2", "value2"), __.has("indexed-prop3", "value3")). Essentially, change the union step into "or" step.
Hope this helps!