HI Dilip, while you could do that, since you’ve only got a single secondary it would be promoted to primary state should something happen to the current primary.
As for having your reporting only querying the secondary node, I’m assuming you’ve set the read preference to Secondary which means that there will be issues should your primary fail and the current secondary gets promoted. At that time you have no secondary to serve those reads.
I would only recommend having different indexes on a cluster that has a hidden secondary member. Since it’s hidden then it can never become the primary and differing indexes are of less concern in that case. However, since you’re in a PSA set up you cannot do this. Is there a reason for going with PSA?