In order to get higher signal level on my pre amplifier to the tv set.
Yes you can, but unless you understand about phasing, and control the impedance (with non-lossy things like a Wilkinson combiner or a quarter-wave transformer (don't use a lossy 6dB equal split combiner (you'll not get any more signal level))) it's unlikely to give you much benefit, and may make things worse.
It's far better to get a longer antenna, that is a Yagi with more directors.
Can I connect two tv antennas in parallel in order to get higher signal level on my pre amplifier to the tv set.
No you can't.
Without an amplifier and with respect to impedance matching to avoid signal losses on cables due to reflections, no you can't. The matching and summing network means that the loss of each individual signal strength due to summing cancels the overall gain in adding two signals together.
A simple 50 ohm combiner uses three 16.7 ohm resistors - when both inputs and the output are connected, each "port" is correctly terminated. Now, if the two input ports were fed from an identical signal sourced twice through two 50 ohm source impedances you would get this: -
If you did the calcs/math you will find that you get exactly the same signal voltage feeding the output 50 ohm (R6) as you would from one source (with its 50 ohm impedance) directly feeding R6 i.e. a signal that is reduced by 6 dB.
You also have to be aware that each antenna has to be positioned so as not to "steal" the power received by the other antenna - you might get lucky with this but that won't help alleviate the situation in my 1st paragraph.