PA32 LANDED ON OCCUPIED RWY HAD LTSS FROM C172. RWY INCURSION. SYS ERROR.

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WHILE WORKING GND CTL; THE PHONE RANG. THE LCL CTLR ANSWERED SINCE NO ONE ELSE WAS AVAILABLE. IT WAS HER DENTIST. I ASSUMED IT WOULD BE A SHORT CALL AND CONTINUED MY DUTIES. THE CONVERSATION CONTINUED FOR QUITE A WHILE. I THOUGHT I SHOULD TAKE THE LCL POS ALSO; BUT DID NOT. THE LCL CTLR CONTINUED WORKING TFC WHILE TALKING TO THE DENTIST. AT 1 POINT; ACFT X EXITED RWY 35L. I ASKED LCL FOR XING ACROSS 'BOTH RWYS' (RWY 32 AND 35R). SHE LOOKED AT ME AND NODDED IN THE AFFIRMATIVE. I ISSUED THE XING CLRNCS AND ACTIVATED THE RID LIGHTS RWY 32 AND RWY 35R (AURAL ALARM BROKEN). I THEN LOOKED AT THE BRITE TO OBSERVE HER TFC FOR A BIT. WHEN I LOOKED AT TXWY K TO OBSERVE THE CHEROKEE X; HE WAS XING RWY 35R. AT THAT MOMENT IN MY PERIPHERAL VISION I SAW A CESSNA Y LNDG ON THE RWY. NEITHER PLT SAID ANYTHING AND THEY BOTH CONTINUED. IN HINDSIGHT: 1) I WAS CTLR IN CHARGE AND SHOULD HAVE ANSWERED PHONE OR LET IT RING. 2) SINCE SHE ANSWERED I SHOULD HAVE TAKEN OVER HER DUTIES AND DEMANDED A BRIEFING. 3) I SHOULD HAVE WAITED FOR AND/OR DEMANDED THE CORRECT RESPONSE PHRASEOLOGY FROM LCL; IE; 'CROSS RWY 32 AND RWY 35R' INSTEAD OF ACCEPTING A NOD OF THE HEAD (SHE THOUGHT I ONLY ASKED TO CROSS RWY 32). 4)ALTHOUGH I SCANNED THE RWY AND APCH END FINAL I FAILED TO SEE THE CESSNA. I COULD HAVE BEEN MORE THOROUGH. SUPPLEMENTAL INFO FROM ACN 294762: I WAS WORKING LCL CTL. I HAD A CESSNA DOING TOUCH-AND-GOES ON RWY 35R. I ALSO HAD A CHEROKEE LAND ON RWY 35L. I TURNED THE CHEROKEE OFF AT TXWY K MIDFIELD AND CONTACT GND. I THEN CLRED THE CESSNA FOR TOUCH-AND-GO RWY 35R. DURING THIS TIME THE PHONE RANG. I WAS WAITING FOR A CALL FROM MY DENTIST. I SHOULD HAVE HAD HIM CALL BACK; BUT I DIDN'T. NOW THE CHEROKEE NEEDED TO CROSS 2 RWYS TO GET TO THE GA RAMP INCLUDING RWY 35R. GND THEN ASKED ME IF HE COULD CROSS; BUT I THOUGHT HE SAID 'CROSS RWY 32.' I SHOOK MY HEAD YES AND SAID 'CROSS.' I ASSUMED HE WOULD HOLD SHORT OF RWY 35R. THE CHEROKEE CROSSED BOTH RWYS WITH THE CESSNA ON FINAL TO RWY 35R. I ASSUMED SOMETHING I NEVER SHOULD HAVE. I ASSUMED GND KNEW I HAD TFC ON RWY 35R AND I THOUGHT HE ASKED TO CROSS RWY 32 ONLY. I SHOULD HAVE USED THE CORRECT PHRASEOLOGY 'CROSS RWY 32 HOLD SHORT RWY 35R.' BUT I DIDN'T.