MTBF / MTTF and parallel systems
My first question would be the following: in many calculations and
  handbook (217F), only FIT ie. MTBF values are accessible. In case on a
  non-repairable system such as mine, can I safely assume that MTTF =
  MTBF?


MTBF = MTTF + MTTR : -



Realistically, what most people mean when they say "MTBF" is "mean time to fail" (MTTF). So, you can't reasonably and safely assume anything. You can't assume MTTR is zero but many people do.


  let's assume components (let's say they are resistors) A and B are in
  parallel (let's assume they share the same load in normal operation)
  the system fails only if A and B fail at the same time. I would like
  to write this but I'm not certain


You need to decide whether it's possible to repair resistor A while resistor B is still working (i.e. the system is not suffering down time). You also need to decide if resistor A failing is self revealing and, if it is you also have to factor-in the reliability of the circuit that reveals resistor A or B failing. See MTBF and MDT for networks of components in the same wiki page as above.