I am assuming the antennas are aimed in different directions, receiving different stations. (If you are attempting to gang them, to make them into a single antenna, take a look at this information on
antenna stacking.)
You can try a reversed 2-way splitter... it's cheap and easy. If it doesn't perform the way you expect, it's easy to back up and start again.
However, there is a reason CATV head-ends have thousands of dollars per channel worth of equipment installed to receive, filter, process and often shift the frequencies. (But if you can get a $6 splitter to work, great.)
Here's what works for me. (Not a one size fits all solution.)
Many use an A/B switch of some type.