Entering Japan on one passport, activating WHV later with another — possible?

Hey everyone, just wanted to check if this plan sounds possible or if I’m missing anything obvious.

I’ve already got a Working Holiday Visa for Japan under my New Zealand passport, but I’m planning to move there in early December.

For the first couple of months, I just want to travel and enjoy Japan rather than start my WHV right away.

So my plan is to:

1.  Enter Japan in December using my British passport (visa-free for 90 days), so I can just be a tourist and enjoy tax-free shopping.

2.  Then, in late January, I’ll take a short trip to Korea for a few days.

3.  When I return to Japan, I’ll enter using my New Zealand passport and activate my WHV properly at that point.

Is this okay to do? I can’t see any big issue since I’d be entering with different passports and following all entry rules, but I wanted to confirm whether anyone’s done this before or if immigration would have a problem with it.