question

You already found the right place to do it. Use the OnEntry Trigger of the Combiner.

To start you also can use the pick option Set Location/Rotation/Size. But from this point you will have to write a little script that says, depending on the item you are, you have to go to a different location. The Pick option gives you the Location stuff. For finding out what part arrived you could use for example the ports the item came in.

So the script could look like:

double d_xpos = 0;//creating local variables
double d_ypos = 0;
double d_zpos = 0;

if(port == 1) // if the item entered object from port 1
{
	d_xpos = 1;//defining the value for x position
	d_ypos = 2;
	d_zpos = 3;
	
}
else //if the item entered object from the other port
{
 	d_xpos = 2;
	d_ypos = 2;
	d_zpos = 2;
}
setloc(item,d_xpos,d_ypos,d_zpos);//set the location to the item that fired the trigger

To find out the right positions, I´m sure you will have to try out.

For a first hint, you can let your model run until the items are both in the Combiner. Click on the item and place them where ever you want them to be and read the values from the Quickproperties or the bottom left corner of the 3D View window

For Rotation and Size the commands are:

setrot(Object,x,y,z);

setsize(Object,x,y,z);