Airlines never seem to want joint lounges unless they are compelled (as at LAX, and even there it took a decade of negotiation). BA has resisted changing terminals at several airports in the past, precisely because of the lounge situation (large investment in existing lounge in the old terminal, problems getting equivalent lounge facilities in the new terminal); looks like NRT has the same problem. Given the BA route network, connecting to JL is much less important for BA than for AA (or even QF).

I wonder where the new QF and CX lounges will be in terminal 2? There doesn't seem to be space adjacent to the new AC, and space is tight in T2 in general (unless they've added some in year 1 of that USD 170 million improvement program).

One reason to have separately run lounges is the ability to accomodate irregular ops. CX will keep the lounge open when flights are delayed, no matter how long it takes. AA will close their lounge on schedule, as will QF. This difference in philosophy is irreconcilable, and CX will always have their own lounge operation because of this.