Hello, I have noticed cars with two lines and some with one like mine, whats the deal with this?

I have an 98 model year with the one line in use but car is equip with the fittings for two but blocked off.

 

Don't know about the radiator connector as I do not have one, but the recovery reservoir extra port is just that. It is blocked and not used.

 

Some use both some dont, I have no explanation as to why this is, but from the pic below you can see this XK does

 

The attached TSB might help to explain this a bit.

I ran into this issue when I replaced the coolant resorvoir on my 97 which had both hoses. The replacement resorvoir had one nipple blocked. So I just connected the hose from the top of the radiator (which is no longer needed according to the TSB) to the blocked nipple. So I still have 2 hoses but one of the hoses is useless other than sealing the connection from the top of the radiator.

This procedure of reversing the hose connections is explained in the last section of the TSB titled "1997 MY VEHICLE USING 1998 MY COOLANT RESERVOIR".

Hope this helps.