But I don't understand for what reason "they may not work well if connected in parallel".
I would not connect arduino input directly - you need another circuit to create larger pulses (as the pulses you get are 0.1V peaks or less). You may use the second circuit with LM311 comparator, powered from 5V, and without the T2 transistor - wire the output from R6 directly to arduino - and count the pulses.
the capacitance of the inactive diode will reduce the amplitude of the pulse signal generated by the active diode.
As the bandwidth is sufficient there is no reduction in amplitude,
And is it possible that you (pito) may make a circuit diagram or something for me
QuoteAs the bandwidth is sufficient there is no reduction in amplitude, That is simply nonsense.