Ok, I hope I understand it right. Then this scenario doesn't look as designed:
(not sure if each step is necessary to reproduce, however..)
have "Allow mutliple connections" activated on PLSD ..1620
have "Store recent history" activated
have "Store with password" deactivted
(all connection changes/toggles below) are done via connection switch, recent history of the only one open sql window)
log off all, leaving one sql window open
log on DB A, enter pw
already open sql window is connected to main connection A
use sql window connection switch to change db connection to B
enter PW for connection B
run query, ok
use connection switch to go back to A
got message: Could not log on as [user]@ConnectionA, ORA-01005: No password..
click ok
get logon dialog for A again
enter PW,
connected
run query, ok
switch sql window back to B
no connection error this time for B
run query, ok
switch again to connection A
got message: Could not log on as [user]@ConnectionA, ORA-01005: No password.. again
reenter PW, run query, ok
switch sql window back to B
no connection error this time for B
...
You can repeat that probably endless. The original,main(!) connection gets never switched back without PW.
I didn't try with more than two connections.