What do you mean by "benefit"? Are you asking whether they could be validated concurrently? For example, you could test the hypothesized factor structure of each scale. You could also look at convergent validity in terms of how strongly the scores on the two questionnaires are correlated (assuming they are supposed to measure the same or similar attributes).

Factor analysis is not generally used for validation, but I suppose that finding the same factor structure in two two separate samples would increase you confidence in those results.

For assessing validity, I think you should look into the components of construct validation, such as convergent validity, discriminant validity, and predictive validity.

whatever the use, always use EFA first then go for SEM (CFA mainly) along with two important validity test Convergent and Discriminate validdity. If you do these then it will be mostly validated

If the original instruments were already validated with both EFA and CFA you must validate with only a CFA to confirm your data are achieving the same or similar results. The factors must stand the same. If you run the EFA and find different factors then there may be a translation issue with the items.