How do I get an NPC party member to stay put in one position, BESIDES making them "hide" and go into stealth?

It's rather annoying when I need to spread out the party to do certain things and every time I click a party member, all of the rest of the party follows along behind the "leader"

For example, I explored the cellar in the Toll House and discovered that by sitting in the 2 ornate chairs, a secret wall opened up to an armory room.

The problem is, the 2 chairs were 20 feet apart and every time I selected one person to sit in a chair, and then attempted to make another NPC sit in the OTHER chairs, the first sitting NPC got up off the chair and ran over to the selected NPC.

So again, how do I make an NPC stand in one place and not follow the group?