You won't break anything. You may get lucky and see an improvement on one or two channels.
BUT, most of the time, the result is like tying two cats to a cart with a pit bull in it... let's just agree that it's random coincidence if the cats pull together at any given moment.
There are tuned filter devices that can be used to join the output of two or more antennas. When the antennas are designed for different bands (UHF{CH14 and higher}, High-VHF {CH 7 through CH-13} or Low-VHF {CH2 through CH-6}) products such as a UVSJ and HLSJ are fairly inexpensive. When trying to join signals in the same band, the quality of the filters needs to be higher which drives up the cost.
http://www.solidsignal.com/pview.asp...nna-%28UVSJ%29
http://www.solidsignal.com/pview.asp...-%28ZHLSJ-1%29
http://www.tinlee.com/MATV_headend.p...IGNALINJECTORS
Then there are other options to consider:
http://forum.tvfool.com/showthread.php?t=2882
http://forum.tvfool.com/showthread.php?t=820