RTXI has no built-in software limitations on the number of DAQ cards. You will need to edit the configuration file, however. Here is the relevant excerpt of /etc/rtxi.conf:

<OBJECT component="plugin" library="analogy_driver.so" id="2" >
<PARAM name="0" >/dev/analogy0</PARAM>
<PARAM name="Num Devices" >1</PARAM>
<OBJECT id="13" name="0" />
</OBJECT>

Edit the lines to add another ANALOGY device and change the number of devices:

<PARAM name="0" >/dev/analogy0</PARAM>
<PARAM name="0" >/dev/analogy1</PARAM>
<PARAM name="Num Devices" >2</PARAM>