The line 1 and line 2 are "separate" lines, but have the same phone number. This lets them have a conversation on one phone line, and make a call (or receive another call) on the other line. I find it VERY convenient when my wife is on the phone and I need to make a phone call.

If you want all the phones to be tied to the same line like you mention above, a 2-1 splitter should work. But they then wouldn't be able to have two calls at the same time like they do now.