I want to use the LSU 4.9 with the "2JZGE NA Header" with EmuBlack.

The "2JZGE NA Header" has one "narrow band oxygen sensor" per 3 cylinders (2 total).

If I build an LSU 4.9 with NA, do I wire both of them to the EmuBlack?

Wire only one of them to use?

It's better to have both sensors connected to ECU but having just one of them would do the job also providing that the engine is healthty state for sure...

Rather surprised it doesn't have dual LAMBDAS - least as far as I can see - yet two EGT inputs, as the former would be more common.

They may have work-arounds, but as Shota said, you may have to just run a single sender - maybe swapping it over as required for testing? It should work reasonably well, especially if you keep an eye on the sparkplugs which may indicate if there's a developing problem for either trio of cylinders.