Yes, there is one shared sense of simulation time, so both objects A and B see the same time values at the same instant.
  For every time the idle function gets called, A will only move a little bit, and so will B (if they both should be moving at the current time).
  Example:
...
new path diamond start
 	0   0  0  0
	4   10 10 2
	7   0  20 4
	12 -10 10 2
	17  0  0  0
end
tell A animationPath diamond
tell B animationPath diamond 3

From time 0 (when the program starts) to time 3, only A is moving.  From time 3-17, both are moving, and from time 17-20 only B is moving.  The whole time, B is following A (and is behind by 3 seconds).  Equivalent behavior could be achieved with the following example:

...
new path diamond start
	0   0  0  0
	4   10 10 2
	7   0  20 4
	12 -10 10 2
	17  0  0  0
end
new path diamondDelay start
	3   0  0  0
	7   10 10 2
	10  0  20 4
	15 -10 10 2
	20  0  0  0
end
tell A animationPath diamond
tell B animationPath diamondDelay