the ht648 and the ht648l
the ht648 is a encoder chip , the address lines act as a coding system so that the decoding chip will only respond to the encoder ic, albeit , there are only 512 possible codes to cycle thru so don't go using this on something expensive , the ht648 does encode up to 8 switches to a serial stream with its code value , the chip you discussed only has 4 switch inputs ,
the difference between a multiplexer and a encoder is that a multiplexor is used to switch multiple singnals tru the same line by switching thru the singnals at least 10 times faster than the highest frequency your transmitting , the demux, or de multiplexor must then switch the same speed and in sync with the other end and the signals will come ou tthe same(ish) the other end . encoders are usualy for encoding data onto a serial or parralel bus ( this being a bus or a ir diode or a rf tx ) mostly they are used to trasmit data between one thing and another.
from what i have seen of the ht648 and the ht648l they are designed for remote controls usage , very low data rate , as in a yes i want to turn a light on or turn it off , they are not likely to work well with hundreds or thousands of switch combinations a second, namely a rf modem or a uP interface between cpu to cpu , if you want to do that then you would be well adviced to get a good book and study the subject in depth before trying to build this as it will most likely not work without a lot of tinkering.
good luck with your project , let me know if this answered your question and if you need any further info, if you let me know what it is your trying to make maybe i could offer you some ideas.
mike