Graham, detect to kill solution might be measured in hours as the datum is refined and the kill system is accurately positioned. OTOH you might have only minutes between detect and a kill shot - 6 minutes say. That would require a Nimrod to be within 15 miles or so of the target.
Using two platforms would virtually require two UAVs in near proximity. The failure of one would mean a mission failure. Oddly a larger MPA can operate successful with a high degree of redundancy.
On sonobouys, and to think we used to carry 63 or occasionally 94 (luxury) but that often meant we were channel limited and had to compromise coverage using X or X+1 is we used a Hellier Box rather than a chevron or X+2 or +4. Anything over X meant we ran the risk of running out of sonobuoys or channels. Some buoys designed to die after an hour or so might stay up for 2 hours and an 8 hour field might still be radiating for over 10 hours. The only way to 'free' up channels was to descend below the radio horizon. That rather scuppers the idea of high level loitering. At least with many more channels now that problem is much reduced.