Both exist and they are not interchangeable. The stack has an order, and the order encodes scope: each particle applies to everything on its left, including the particles already there.
The usual shape is attitude first, politeness last. นะ, สิ, ล่ะ, หรอก come before ครับ or ค่ะ.
- ไปนะครับpaj ná kʰráp
I'm off then
soften the sentence, then say the softened thing politely
- ไม่ใช่หรอกครับmâj tɕʰâj rɔ̀k kʰráp
no, not quite
the week 13 softener, made polite
So what does ครับนะ do
It reverses the scope: the นะ now applies to the ครับ, which makes the politeness itself the thing being softened. The result is warm and slightly coaxing, the tone of someone reassuring you or seeing you off.
It is a real thing to say and it is not a general-purpose swap. If you are unsure, the attitude-then-politeness order is the one that is always safe.
The useful side effect
Three-particle stacks are common in speech and you do not need to produce them for a long time. What you need is the shape, because it helps you listen.
The politeness particle is almost always the right edge. When you hear ครับ or ค่ะ, the sentence is over, and that is a reliable boundary in a language that gives you very few of them.
- ครับkʰráp
the right edge of a male speaker's sentence
- ค่ะkʰâ
the same, female speaker