Just something to try, basically using nested If functions for the four different scenarios:
1 empty, 2 empty: show the whole list
1 filled, 2 empty: Your original code
1 empty, 2 filled: Filter for status
1 filled, 2 filled: Combining both above
If(
And(IsBlank(FilterBox.Text), Or(IsEmpty(ComboBox1.Selected), IsBlank(ComboBox1.Selected))),
Helpdesk,
If(Or(IsEmpty(ComboBox1.Selected), IsBlank(ComboBox1.Selected)),
SortByColumns(Filter(Helpdesk,( StartsWith(Requestor.DisplayName , FilterBox.Text)) Or (StartsWith(Subject , FilterBox.Text))),"Request_x0020_Date",Descending),
If(IsBlank(FilterBox.Text),
SortByColumns(Filter(Helpdesk,(Status=ComboBox1.selected.value), "Request_x0020_Date",Descending),
SortByColumns(Filter(Helpdesk,(Status=ComboBox1.selected.value) And ( StartsWith(Requestor.DisplayName , FilterBox.Text)) Or (StartsWith(Subject , FilterBox.Text))),"Request_x0020_Date",Descending),
)
)
)