Same here - one boom, one contrail, and only one aircraft on the screenshots of the tracking site...
Lots of military/security situations have 2 parties operating in cooperation; one that the antagonist (and casual observers) can see, and one that is more covert.
One half of the team (policeman/nightclub-doorman/patrol) makes itself known to the target, attempts to determine their intentions, and seeks to influence their behavior. Because of their observability and proximity, however, they are not necessarily best placed to respond to aggressive or non compliant actions by the perpetrator.
The other half of the team (policeman/ambush party/interceptor) stands back, normally carries a big stick, and prepares to intervene if required. If you've noticed him, he's lost part of his advantage.
Rather than ask why you couldn't see the second Typhoon, perhaps we should be considering why we
could see the first? Why would a military aircraft on a live operation employ the tactic of making itself visible on a tracking website and (presumably) on TCAS?