Legal action about job change from one psu to another psu

When I was private employed, I was appeared in the interview of two different unit of same central govt. PSU (BPCL) {let the name of 1st unit be 'X' and 2nd is'Y'} and got selected in 'X'. I had joined 'X' with all the basic formalities. 'X' has issued my joining letter after some days of joining, but after 5 months of joining I had got selected in 'Y' also. I had decided to join 'Y'. Therefore I had resigned with 'X' on personal reason, in my resignation letter I had not disclosed about my selection in 'Y'. And taken proper relieving letter from X. And then i joined Y, but I had decided not to disclose my previous employer detail in attestation form (by ignoring the warnings of attestation form). Because i was employed in same psu different unit and my selection was completed on the past private employer experience. My question is:- (1)If by any means 'Y' came to know about my previous employment with 'X', can any disciplinary action against me will be initiated while i have proper release order from 'X'.?(2) What will happen if I will submit proper release order after any action against me??? (4)Does 'Y' can start enquiry on anonymous complaint.