BOS TWR DEPARTS ACFT Y AS ACFT X IS XING THE SAME RWY.

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NORMAL APCH AND LNDG ON RWY 22L; AT MINIMUMS; LIGHT SNOW; FOG WE CLRED THE RWY AT SIERRA. WE WERE TOLD TO HOLD SHORT OF RWY 22R; CONTACT TWR ON 128.8. CAPT INSTRUCTED ME TO CONTACT COMPANY AND FIND OUT WHERE GATE XX WAS; BECAUSE IT WAS NOT ON OUR CHARTS; AND TO LET MAINT KNOW WE HAD A RADIO ALT #1 FAULT ON LNDG. I LEFT THE FREQ MOMENTARILY TO COMPLETE THE TASK. (WE WERE STOPPED HOLDING SHORT AT THE TIME). AS I CHKED BACK IN WITH THE CAPT; WE WERE CLRED TO CROSS RWY 22R AND CONTACT GND ON 121.9. I ONLY CAUGHT THE LAST PART OF THE ATC XMISSION; SAYING TO 'CONTACT GND 121.9' THE CAPT ACKNOWLEDGED THE ATC CALL AND I ASKED IF WE WERE CLRED TO CROSS. HE SAID YES; AND WE PROCEEDED TOWARDS THE RWY. IT LOOKED AS THOUGH THE TFC WAS ROLLING ON RWY 22R. SO I ASKED THE CAPT; 'IS THAT GUY ROLLING?' THE CAPT APPLIED THE BRAKES; BUT I GUESS DUE TO AN OPTICAL ILLUSION; WE BOTH AGREED HE WAS STATIONARY. BUT AS WE CROSSED THE RWY; THE OTHER ACFT ROTATED AND PASSED OVERHEAD. HE WAS APPROX 3500 FT AWAY WHEN HE LIFTED OFF AND 400 FT ABOVE US. NEITHER THE CTLRS OR OTHER ACFT SAID ANYTHING; WHICH LEFT US ALL PUZZLED. I SUGGESTED THE CAPT CALL THE TWR; BUT HE SAID THEY'LL CALL US IF THEY NEED US. ABOUT AN HR LATER; WE DEPARTED; AND ON TAXI OUT WE WITNESSED ALMOST THE EXACT SAME THING WITH ANOTHER ACFT XING RWY 22R. EVEN THOUGH THESE XINGS ARE AT THE END OF AN ACTIVE RWY; IT IS STILL VERY DISCONCERTING TO SEE ANOTHER ACFT BARRELLING DOWN THE RWY TOWARDS YOU. I DO NOT KNOW IF THIS IS A NORMAL PROC AT BOS; BUT IT SURE HAD MY ADRENALIN PUMPING. ABSOLUTE VIGILANCE ON RWY XING IS A MUST; AND A SET OF RULES SHOULD BE IN PLACE SO THAT AN ACFT IN POS SHOULD HAVE NO LIGHTS ON; AND ONE ON THE ROLL SHOULD HAVE ALL LIGHTS ON. THIS WAY AN APCHING ACFT WILL KNOW IF ONE HAS BEEN CLRED TO DEPART. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THEIR ACFT STAYED ON TXWY SIERRA (S) ALL THE WAY ACROSS TO THE RAMP. HE ALSO STATED; THAT BECAUSE OF DARKNESS; SNOW AND LOW VISIBILITY AT THE TIME; THEY WERE UNSURE IF THE OTHER ACFT ON RWY 22R WAS ACTUALLY IN ITS TKOF ROLL OR NOT. RPTR CLAIMS; ONCE THEY REALIZED THAT THE OTHER ACFT WAS TAKING OFF; THEY INCREASED THEIR TAXIING SPEED; BUT IT WAS TOO LATE; AS THE OTHER ACFT WHICH APPEARED TO BE COMMERCIAL JET; PASSED DIRECTLY ABOVE THEM.