gain of identical antenna
There are a number of issues to consider. Combining antennas can increase the gain and you generally get a 3 dB increase, or a bit more if you do it correctly. You can also hurt the situation if you make an error. You are probably bette off with a single antenna.
You might want to consider other lower loss coax options. Generally the bigger the coax diameter, the lower the loss (and the higher the cost).
An amplifier immediately after an antenna can help overcome cable loss but again you have to pay attention to the details. It is probably a good idea to add a bandpass filter in front of your amplifier to reduce the chance of saturating your amplifier with nearby but out of band signals. Look into LNA's (low noise amplifiers).
Amplifier noise figure, gain linearity, third order intercept, power consumption, cost etc. are additional factors to consider.
Various trades are more easily understood if you do a "link budget" analysis.