8.6.1.1090
D2*
d-two-star
DOUBLE
 
( xd1 -- xd2 )

xd2 is the result of shifting xd1 one bit toward the most-significant bit, filling the vacated least-significant bit with zero.

Testing:
T{              0. D2* -> 0. D2* }T
T{ MIN-INT       0 D2* -> 0 1 }T
T{         HI-2INT D2* -> MAX-2INT 1. D- }T
T{         LO-2INT D2* -> MIN-2INT }T