Sea local controller reported an aircraft executing a missed approach was involved in an near midair...

Narrative:

Aircraft X was on final to runway 16R and was just passing over bfi. As I was monitoring the scope due to multiple go arounds [from] inadequate spacing on final causing compression; I noticed aircraft Y go around from bfi. The targets merged and were showing identical altitude read outs. Sea tower never received notification from the shout out line as pertained in the LOA (letter of agreement) between the three facilities. TRACON to my knowledge transferred communication to the local control and aircraft X landed. Aircraft X had gone around previously due to compression on final. This is a prime example of how to my knowledge there is no solid protection between the two finals in regards to sea and bfi. TRACON does not stagger arrivals or protect sea final from bfi go arounds. The current protection that bfi's go around won't break a certain altitude is not enough and allows too much possibility for error that a pilot will break it when they're already responding to a heavy task load situation.bfi traffic intruding on sea final traffic is only happening more frequently. Aircraft on final to sea should have solid protection on the final approach corridor. Sea class bravo airspace has already been reduced to the smallest footprint possible to be 'VFR friendly.' safety needs to be the top concern.

Original NASA ASRS Text

Title: SEA Local Controller reported an aircraft executing a missed approach was involved in an NMAC with another aircraft on final approach to SEA.

Narrative: Aircraft X was on final to Runway 16R and was just passing over BFI. As I was monitoring the scope due to multiple go arounds [from] inadequate spacing on final causing compression; I noticed Aircraft Y go around from BFI. The targets merged and were showing identical altitude read outs. SEA Tower never received notification from the shout out line as pertained in the LOA (Letter of Agreement) between the three facilities. TRACON to my knowledge transferred communication to the Local Control and Aircraft X landed. Aircraft X had gone around previously due to compression on final. This is a prime example of how to my knowledge there is no solid protection between the two finals in regards to SEA and BFI. TRACON does not stagger arrivals or protect SEA final from BFI go arounds. The current protection that BFI's go around won't break a certain altitude is not enough and allows too much possibility for error that a pilot will break it when they're already responding to a heavy task load situation.BFI traffic intruding on SEA final traffic is only happening more frequently. Aircraft on final to SEA should have solid protection on the final approach corridor. SEA Class Bravo airspace has already been reduced to the smallest footprint possible to be 'VFR friendly.' Safety needs to be the top concern.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1702268
Time
Date	201911
Local Time Of Day	0001-0600
Place
Locale Reference	SEA.Tower
State Reference	WA
Aircraft 1
Make Model Name	Large Transport
Operating Under FAR Part	Part 121
Flight Phase	Descent Final Approach
Flight Plan	IFR
Aircraft 2
Make Model Name	Medium Transport
Operating Under FAR Part	Part 135
Flight Phase	Climb
Flight Plan	IFR
Person 1
Function	Local
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (yrs) 1.9
Events
Anomaly	ATC Issue All Types Airspace Violation All Types Conflict NMAC Deviation - Procedural Published Material / Policy

Sea local controller reported an aircraft executing a missed approach was involved in an near midair collision with another aircraft on final approach to sea.