If both stb has deferent address && phone no, it's not possible.
So if u want to merge both stb's in to one cust ID kindly go through given below steps.
1.pls ensure that both ID has same contact name/ no & address with pin code.
2.decide which stb you want as primary,secondary*.
3.write to airtel/distributor/tsm. If TSM(service manager) forward this mail to supportdth it will be get merged in to ur 1 existing ID (3,4 working days).
*send mail with all details.... i have 2 connection in my house with deferent ID's(specify ID's), i want to merge it. Pls add stb no-025544565(here it's cust ID) as 2nd connection under 301434534XX-001