Repeat something exactly twice, then stop

["Andrew Koenig" <ark@acm.org>]

This isn't quite a loop, but I think it's close enough.

I want to be able to play something and have it repeated after a delay of
about 15 seconds, then have it repeated again after a second delay of the
same length, then not have it repeated further.  I want this delay to be
continuous: That is, I want to be able to keep playing and hear, mixed with
what I'm playing, what I played 15 seconds ago and what I played 30 seconds
ago.

I have a Boss DD-20, which can repeat something after a 15-second delay, 
but
only once.  Or it can repeat something twice, but the first delay cannot be
as long as 15 seconds.

I had thought that I could use the DD-20's stereo delay as two mono delays:
Connect the input to one channel, connect that channel's output to the 
other
channel's input and use that other channel's output.  But that causes the
second channel to delay *both* signals, which means that the one in the
middle gets played twice.

Any other suggestions, other than getting a second delay unit?