Light Transport; Low Wing; 2 Turbojet Eng
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 20
Confusion; Distraction; Situational Awareness
ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Clearance
Aircraft; Human Factors; Procedure
Aircraft X landing SDL.  Aircraft Y landing IWA.  I coordinated a stack (SDL lander at 11000 and the IWA lander at 12000).  LOA calls for both aircraft to be at 12000 and issued descent to aircraft 1 to 14000.  This aircraft had to level at 14000 due to the proximity of NAA airspace (13000 and below).  I issued the IWA inbound a descent to 15000.  When SDL inbound was clear of NAA; I issued 'descend and maintain 11000'.  When I observed the bottom aircraft descending out of 12800; I issued a descent to the IWA inbound to 13000.  I then went back and told the SDL inbound (the bottom aircraft) to cross SHOCK at and maintain 11000 and 250knots. When I observed the bottom aircraft descending out of 11800; I issued 'Cross SHOCK at and maintain 12000 and 250knots;' to top aircraft.  At SHOCK intersection; I observed the top aircraft at 12800 descending to 12000 and the bottom aircraft at 11300.  I asked the bottom aircraft to say altitude and he replied 11300.  I told him to increase his descent and he replied 'Traffic insight.'   I used the rule of observing an aircraft leaving an altitude before issuing the altitude to another aircraft; but I am second-guessing my application of the rule because I went back and issued crossing restriction after I issued the descent.My recommendation would be to educate the workforce on when they can and cannot use the rule 'Observed leaving an altitude.'
ZAB Controller uses altitude leaving rule and has a loss of separation.