Welding of P91 to P11

Dear concerns,
I have a doubt regarding to welding of P91 to P11.
My question is that When i reviewing the client drawing i have seen one joint which is P91 to P11 with PWHT. I think is it possible because there is much difference in PWHT temperature of both.please suggest. It is possible?If possible then how? If no then why as per metallurgical way.
Otherwise I will take deviation for placing the pup up piece of P22 in between P91 and P11. please suggest.

As the PWHT temperature of P91 say 760 degC may exceed the lower transformation temperature of P11, a transition piece is recommended. Else butter the P91 portion using E9018 B3, perform the PWHT at 760 for 1 Hour then weld the buttered portion to P11 using E8018 B2 and perform PWHT at 690degC.

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Sir,
For buttering we have to qualify new PQR ?

Yes you need