He reviews their book Fragile by Design.

the observed interdependence of states and banks isn’t as deep-seated and inescapable as Calomiris and Haber claim. Consequently, keeping bankers and governments from getting too cozy with one another isn’t quite so difficult as they suppose.

Later, Selgin writes,

they seem unaware of the adverse effects of the “bond-deposit” provisions included in misnamed state “free banking” laws. These provisions allowed banks to issue notes only after tendering eligible securities to state authorities for the ostensive purpose of securing the notes’ holders from loss. Calomiris and Haber (p. 169) note that, by making their own bonds eligible for this purpose, states were able to force banks to lend to them “in exchange for their right to operate.” Still they fail to point out that some states force-fed their banks, not “high-grade” bonds (ibid.) but junk ones, and that it was this practice, rather than unit banking, that was the main cause of bank failures during the so-called “free banking” era

…In Canada, in contrast, banks’ almost unrestricted ability to issue notes
contributed to the banking system’s stability no less than banks’ branch networks did.

You may also wish to read my review of the book.