How to set several artLayers as activeLayer at the same time?

You can select layers by Index , Id or by Name

Or select all layers.

Here are some functions that will let you do all the options including the merge.

function selectLayerByIndex(index,add){ 
var ref = new ActionReference();
ref.putIndex(charIDToTypeID("Lyr "), index);
var desc = new ActionDescriptor();
desc.putReference(charIDToTypeID("null"), ref );
if(add) desc.putEnumerated( stringIDToTypeID( "selectionModifier" ), stringIDToTypeID( "selectionModifierType" ), stringIDToTypeID( "addToSelection" ) ); 
desc.putBoolean( charIDToTypeID( "MkVs" ), false ); 
try{
executeAction(charIDToTypeID("slct"), desc, DialogModes.NO );
}catch(e){}
};
function selectLayerById(id,add){ 
var ref = new ActionReference();
ref.putIdentifier(charIDToTypeID("Lyr "), id);
var desc = new ActionDescriptor();
desc.putReference(charIDToTypeID("null"), ref );
if(add) desc.putEnumerated( stringIDToTypeID( "selectionModifier" ), stringIDToTypeID( "selectionModifierType" ), stringIDToTypeID( "addToSelection" ) ); 
desc.putBoolean( charIDToTypeID( "MkVs" ), false ); 
try{
executeAction(charIDToTypeID("slct"), desc, DialogModes.NO );
}catch(e){}
};
function selectLayerByName(lyrName,add){ 
add = undefined ? add = false:add 
var desc = new ActionDescriptor(); 
var ref = new ActionReference(); 
ref.putName( charIDToTypeID( "Lyr " ), lyrName); 
desc.putReference( charIDToTypeID( "null" ), ref ); 
if(add) desc.putEnumerated( stringIDToTypeID( "selectionModifier" ), stringIDToTypeID( "selectionModifierType" ), stringIDToTypeID( "addToSelection" ) ); 
desc.putBoolean( charIDToTypeID( "MkVs" ), false ); 
executeAction( charIDToTypeID( "slct" ), desc, DialogModes.NO ); 
};
function selectAllLayers() {
var desc29 = new ActionDescriptor();
var ref23 = new ActionReference();
ref23.putEnumerated( charIDToTypeID("Lyr "), charIDToTypeID("Ordn"), charIDToTypeID("Trgt") );
desc29.putReference( charIDToTypeID("null"), ref23 );
executeAction( stringIDToTypeID("selectAllLayers"), desc29, DialogModes.NO );
};
function mergeSelected() {
try{
executeAction( charIDToTypeID('Mrg2'), undefined, DialogModes.NO );
}catch(e){}
};