Having just done an ex-EU to the US on the return you only need to book one Day 2 test (LFT or PCR as it may be), you need to select transit option in the Passenger Locator form (aka. travel authorisation doc on VeriFly):
JFK/US XXX-LHR: vaccination proof, PLF/travel authorisation form (<48hrs before arrival, transit option selected without passing UK border control) upload to Verifly
LHR-XXX: vaccination, PLF (if relevant for your destination), negative pre-departure covid test (if relevant for your destination), upload to Verifly if available for your destination
XXX-LHR (final leg of B2B): vaccination, PLF (stay in the UK, which will trigger a series of further menus including Day 2 test booking ref number), upload to Verifly