Can two batteries be connected both in series and parallel using an isolated 1:1 DC/DC converter?
So would it be possible to connect an isolated 1:1 DC/DC converter to
one of the batteries and connects its Vout+ and Vout- to the + and -
of the other battery?

Step in the buck regulator - feed it from your 14.8 volt (series battery) and it should very efficiently step down to provide you with a lower voltage supply. It'll be more efficient than an isolating DC-to-DC converter because it won't use a lossy transformer.
In addition, trying to parallel the output of a DC-to-DC converter with an existing battery is going to require careful design to ensure load sharing. Don't bother, just use a buck regulator as previously mentioned.