Huh, I thought it was from the episode after Shotaro used HeatTrigger's Twin Maximum and messed up his body. As for the dialog; I thought it was something more like this;

W: We're going to do a Rider Twin Maximum
Accel: That's a thing?
W: Yep

*edit

I just remembered what the Invisible arc is (although calling it an arc when it's only two episodes long doesn't really seem right to me) and that the Twin Maximum stuff happened during it