
I wasn't thinking about the walking which shouldn't be a problem, more a case of mental exhaustion than physical - I am well into museums, have a degree in archaeology & ancient history, and have led museum tours (not boasting, just pointing out that I have a real deep rooted interest and good 'museum stamina') - and even I just glaze over after a while, there comes a point when concentration goes and you simply can't take any more in! I love the BM but just think it's best to spread it over 2 days rather than try to cram in too much in one go - the place is huge, as is the Natural History Museum (another old favourite), it won't be possible to see much of either in a half day visit each, even if they run around the place.
If it has to be 2 museums in one day, it's a good idea to pick 2 or maybe 3 galleries of particular interest from each museum in advance, rather than wander round feeling pressed for time and then finding that you've missed something you would have liked to have seen - so I'd advise them to have a gander at the respective websites and pick a few things from each museum that they consider 'must-see' (whether that's a particular set of artefacts, or a particular era) and plan their visit around those. Just wandering round they'll see lots of interesting things, but because they won't have nearly enough time to get round the whole museum they're likely to end up sitting down with a coffee later realising 'oh bugger, we didn't get to see the Mildenhall Treasure' or something of that nature.