Combining antennas that operate in entirely different frequency ranges is fairly simple and inexpensive. The common example is a UHF only antenna combined with a VHF only antenna via a UHF/VHF signal combiner such as this
http://www.radioshack.com/product/in...ductId=2103923 or this
http://www.amazon.com/Antennas-Direc.../dp/B008PBTPN4 These devices contain some fairly simple filters that prevent the two dissimilar antennas from interacting/interfering with one another.
Combining two antennas that operate in the same frequency ranges is possible, but requires much higher quality ($$$) filter networks or active electronics ($$$$$).
You can try using two identical antennas and a 2-way splitter in reverse... IF you're very lucky, it might work. Most folks that try this find the results to be disappointing at best. The reversed splitter does not prevent the two antennas from acting as interference sources to one another.
If you need an amplifier, it will do the most good when mounted close to the antenna, before the signal is degraded by cable or splitter losses.