Scott. Yes you can set this up with the 2k. One of the Yamaha demonstrators had added a lower keyboard (PSR290) to the Tyros at the 7 day Keyboard Festival this past April. I am thinking of doing this with my 9000Pro.
(The PSR290 has now been replaced by PSR292. I can get this discount in UK for £169 Pounds).
No sounds are used from the 290, so the board can be placed in a shelf arrangement under the upper keyboard so just the keys are showing (we don't need to get at the buttons) The Acc & lower voice of the upper board can be triggered anywhere along the full length of the lower board plus you can use the full length of the upper board for right hand voices.
I like the idea of making a double manual board for very little outlay. I have a picture somewhere if I can lay my hands on it of Tyros + 290