Hi Jacek,

I assume, you want to count the low and the high time of your input frequency.
Then there are some logical mistakes:

You must wait, until a high to low transition occur to start the low time measuring and stop after the low to high transition to avoid counting the next low period.

Same for high time measuring.

Also you can use the gate function of the timers.

Also you can save the inverter, with 1 timer gated to measure the high time and the other timer not gated to measure the period time:
low time = period time - high time



Peter