The effect can be worse than the impedance mismatch. Depending on the separation distance, the antenna patterns will overlap, and not necessarily in "constructive" ways. You may get destructive interference in some directions. So don't expect great results by just "tee-ing" the antennas together. To get the pattern you want, you'll need to insert a different phase via one of the coaxial lines going to at least one of the antennas (by trying different lengths of coax cable). It can be time-consuming, and typically you won't have any reasonable way of measuring your results...unless you pay very close attention to signal levels from desired stations, and you have a way of seeing or hearing that signal strength change reliably.
At a minimum, try to use a proper power combiner/splitter, like this one from Mini-Circuits:http://www.minicircuits.com/pdfs/ZFSC-2-1.pdf
At least with that combiner you know you have the proper isolation between RF ports to start a proper evaluation.
But all that being said, sometimes you can luck out and get something that was better than what you started with.