When placing pistons horizontally adjacent but vertically one level apart, the upper piston won't retract until the lower one does.

In the case I tested this I used three pistons:

side view:

• - - - - -
• - - - - -
  r R S S S -
  S S S P P -
• - - P'S'-
  S S S S S -

top view:

r R S S S -

• - - R R -
• - - r r -

r = Redstone Wire
R = Repeater
S = Sandstone
S'= Sandstone with Repeater and Redstone Wire in front of it (see top view)
P = Sticky Piston
P'= Sticky Piston with Repeater and Redstone Wire in front of it (see top view)
T = Extension of an activated piston

• = Air

I then activated first the lower repeaters then the upper one:

• - - - - -
• - - S S -
  r R S P T -
  S S - T P -
• - - P'S'-
  S S S S S -

• - - S - -
• - - T S -
  r R S P T -
  S S - T P -
• - - P'S'-
  S S S S S -

The upper piston will now only retract if all repeaters are deactivated.