We are unaware of any evidence to suggest that two consecutive days of assessments are detrimental. To be clear, this includes both assessments being fast assessments, e.g a 6 minute assessment followed by a 3.2 km assessment. Most research protocols using these of similar ‘time to exhaustion’ assessments mandate 24 or 48 hours rest between, though we know of no evidence comparing or evidencing this rationale.
One aspect we are aware of is that research suggests that 3 days of exhaustive time trials induces adverse immune responses, and so in general 2 days is the recommended limit. How this relates to non-exhaustive speed work can only be surmised.
Another aspect to be aware of is that within TrainAsONE the purpose of fast assessment runs is not exactly as often envisaged. It is thought by most people that they are to measure your best performance. However, in reality they are to measure your best performance given your running history. An important, yet subtle difference. So if you perform a 6 minute assessment one day, you are not ‘expected’ to complete a 3.2 km assessment the next day in the best possible time you could (compared if you had had a day or two of rest).