zopffa:
so in fact I write Bit0 and Bit1 value = 1 to the register.Is that what you mean?
Yes.
It may not hurt in this case, because all the bits belong to the same timer,
but it is inefficient (one read and the or are superfluous) and incorrect.
There are some other similar behaving bits, for example the PINx registers,
where writing a 1 to a bit toggles the output state, where it is more critical to get it right.