Any negatives using a Triac to transmit square wave?
Yes, I can think of a good alternative that is better than both a relay and a triac.

You seem to want to be able to select a unidirectional (slave to master) logic level bitstream.from 2 different sources. If you were to connect them together, you will get undefined results because of the output type (not open-drain).

You can use a number of different devices like transmission gates, discrete bipolars, and many other solutions but I suggest a digital switch (multiplexer/demultiplexer). Since you do not need linearity, these should exist in a standard logic catalog. Here are a few examples of analog multiplexers:


TS5A9411 - 10 ohm SPDT up to 5 volts
SN74LV4051A-Q1 - 1:8 MUX if you have 8 desired input sources
Let's Go Shopping on Maxim-IC's site!
Obligatory 74-series logic example


All of these options have multi-megahertz bandwidths so you should be good to at least 100kbaud