It certainly seems to have been well researched - though I still struggle a little with doing both tests on one day. One off day and thats it!
I too have strong concerns with both tests on the same day
If they wanted to avoid too much stress of having a week between tests, perhaps one on a Tuesday and one on a Thursday that same week would be better. Near enough to get the test over with and the marking completed in time; but allowing for that off day! Let's face it we all have off days and when we do ... might as well forget it!
The questions/answers sheet refers to the fact that children are used to sitting KS2 tests on the same day ... however those results will not affect the type of school that the child will be attending for the next 7-years the 11+ does!
After just going through the 11+ with my DC, 1st test just didn't feel prepared - an off day ... had the parent/child chat about it not mattering and that they had another chance. 2nd test went in confident and as a result couldn't have got a higher score!