This isn't going to give you the subsearch you were looking for but I believe it can solve your issue.
[base search] earliest=-14d@d
| eval last_week_start_time=relative_time(now(),"-7d@w0")
| eval this_week_start_time=relative_time(now(),"@w0+1d")
| eval last_week_end_time=relative_time(now(),"@w0")
| eval this_week_end_time=relative_time(now(),"now")
| eval weekday=strftime(now(),"%A")
| eval start_time=if((weekday="Monday" OR weekday="Sunnday"),last_week_start_time,this_week_start_time)
| eval end_time=if((weekday="Monday" OR weekday="Sunnday"),last_week_end_time,this_week_end_time)
| where _time>=start_time AND _time<=end_time
... View more