Hii,
There is no such particular format for CS consent letter for appointment as whole time CS in both holding & Subsidiary company. You can take individual consent letters,i.e, 2 letters for holding and subsidiary respectively.
Pl note that Only in the BM minutes and agenda item for the aforesaid item, there you can mention the relationship citing reference of section 203, pursuant to which Mr. X has been also appointed as the CS of ......Ltd.(Subsidiary Company), who is also a CS (KMP) in ....Ltd. (Holding Co.) and the Board to take note of the same.
Thanks & Regards.