Comment by Taniwha

I think it's more likely that the data needs to cross a clock boundary and multi-clock synchronisation takes time (especially if metastability is potentially an issue) - they way you make it safe is by holding the data you're synchronising stable for multiple clocks and synchronise a strobe signal across the clock boundary and then back again

My guess is that the circuit they're using gets confused if you start a second write when the first is not yet done