NetworkDays() function will only return complete days, i.e. integers. So you won't be able to do a more granular calculation only using this function. You'll need to consider the time of your timestamps by your own, as I showed above.
Here is an adaption to 24 hour working days:
SET TimestampFormat='DD/MM/YYYY h:mm:ss[.fff] TT';
Let vHol = Num(Makedate(2015,12,25)); // just an example
// 24 hours per workday, for all day inbetween the period, excluding bounderies
,if(NetWorkDays(DT2,DT2,$(vHol)), frac(DT2) ,0) // working hours last day
,if(NetWorkDays(DT1,DT1,$(vHol)),1 - frac(DT1) ,0) // working hours first day
,if(NetWorkDays(DT1,DT1,$(vHol)) and floor(DT1)=floor(DT2), -1) // correct for first equals last day