What does it mean for N1/N2 to float?
Float just means that that value will vary depending on ambient temperature pressure etc. It is not controlled. E.g. on n2 control, if n2 goes down then the control puts in more fuel and if n2 goes up then the control puts in less. But if n1 changes (maybe because ambient temp changed ) the control does nothing. If n1 goes down then so be it.