Master-detail combinations of Interactive Grids have a one-to-many relationship, when defined in Apex using the "Master Region" option.
So if you select more than 1 row in the master IG, the detail IG will show "Select 1 row in the master region" instead of all the detail rows for all the selected master rows.

The solution below fixes this. Nothing new, I copied this from a number of sources.
But as usual I could not find a clean and easy example anywhere, so here's my attempt :-)

The idea here is that we do not set the "Master Region" for the "Emp IG - multiple" region, but instead fill a page item "P1177_DEPT_ID_SELECTED" with all the DEPT rows that are selected, and then use that in the Where Clause.
I have disabled all the add/update/delete options for the "Emp IG - multiple" region in this example, but those will work fine using this method.

Note: The dynamic action event "Selection Change [Interactive Grid]" doesn't always do what I expect it to.
For example in Master-Detail situations, when you edit data in a Master row it does not always trigger - so be aware of that possible hiccup.

Region

Identification
	Sequence	-979
	Title	Dept IG
	Type	Interactive Grid
Source
	Source Type	NATIVE_IG
	Table Name	DEPT

Item

Identification
	Sequence	10
	Name	P1177_DEPT_ID_SELECTED
	Display As	Text Field
User Interface
	Region	Dept Items
Read Only
	Read Only Condition Type	Always

Item

Identification
	Sequence	20
	Name	P1177_DEPT_ID_CURRENT
	Display As	Text Field
User Interface
	Region	Dept Items
Read Only
	Read Only Condition Type	Always

Region

Identification
	Sequence	-959
	Title	Emp IG - standard
	Type	Interactive Grid
Source
	Source Type	NATIVE_IG
	Table Name	EMP
Master Detail
	Master Region	Dept IG

Region

Identification
	Sequence	-949
	Title	Emp IG - multiple
	Type	Interactive Grid
Source
	Source Type	NATIVE_IG
	Table Name	EMP
	Where Clause	deptno in ( select t.column_value from table(apex_string.split(rtrim(ltrim(:P1177_DEPT_ID_SELECTED, ':'), ':'), ':')) t )
	Page Items to Submit	P1177_DEPT_ID_SELECTED

Dynamic Action

Identification
	Name	dept_ig_select
Execution Options
	Sequence	10
When
	Event	Selection Change [Interactive Grid]
	Selection Type	Region
	Region	Dept IG
Advanced
	Event Scope	Static
True Action
Identification
	Action	Execute JavaScript Code
Settings
	Code	var i , i_deptnos = ":" , i_deptno , model = this.data.model; for ( i = 0; i < this.data.selectedRecords.length; i++ ) { i_deptno = model.getValue( this.data.selectedRecords[i], "DEPTNO" ); i_deptnos += model.getValue( this.data.selectedRecords[i], "DEPTNO" ) + ":"; } apex.item( "P1177_DEPT_ID_SELECTED" ).setValue (i_deptnos); apex.item( "P1177_DEPT_ID_CURRENT" ).setValue (i_deptno);
Execution Options
	Sequence	10
	Fire On Initialization	No
True Action
Identification
	Action	Refresh
Execution Options
	Sequence	20
	Fire On Initialization	No
Affected Elements
	Selection Type	Region
	Region	Emp IG - multiple