What does two 'high-low' chimes in the cabin of a 737-800 mean?
As far as I know, the chimes work the same on all 737 NG (and Classic) versions, although as was mentioned in comments, how they are employed will vary from airline to airline.

The two tone "high-low" chime described in the OP is the "crew call" system. The pilots have a button, the forward F/A station has one, and the aft F/A station has one.  Regardless of which button is pushed, the "hi-low" chime is the same.  In many, though not all, cases, this chime is a signal to the F/A's to answer the interphone.  If the pilots press their button, they're asking to speak to the F/A's; if one F/A station presses the button, they're asking to speak to the other F/A station.  (They can also press a different button that calls the cockpit, but passengers won't hear that chime. In the cockpit, we hear that "call" as the two-tone chime; we don't hear the chimes in the cabin unless we're listening to PA audio.)

At one carrier, a single high-low chime may mean "answer the interphone," while at another carrier, one chime might be used to mean "secure the cabin" while two chimes is "answer the interphone," three chimes is "answer the interphone - urgent," and so on.

You'll also hear chimes when the F/A Call Button above a passenger row is pressed; this is a distinct chime (a single, relatively high note).  There is also a chime when the Fasten Seatbelt sign is turned on or off, this is again distinctive, a single lower note.

Note that you won't necessarily hear the chimes when the interphone is in use; it's simply a "Hey!" call to the Flight Attendants in the cabin.  While it's often followed by a conversation on the interphone, they two are separate systems and one can be used without the other.