Signal Multiplexing and Bits
No, that's a bit off track. 

Wikipedia's Multiplexing definition sums it up fairly well.


  In telecommunications and computer networks, multiplexing is a method by which multiple analog or digital signals are combined into one signal over a shared medium. The aim is to share a scarce resource. For example, in telecommunications, several telephone calls may be carried using one wire.




Figure 1. Time division multiplexing (TDM) is one flavour. Source.



Back to your question:


  Assume I am working with a 10 Hz, 16 bit message. 


You need to separate the sample rate (how many times a second you need to transmit a value) from the data rate of the carrier (how many bits per second the carrier can handle). The way you have defined this sounds like you want to transmit a 16-bit message 10 times per second. This means that you need to be able to transmit at least 160 bits per second.


  If I have two quantities, A (12 bits) and B (6 bits), I immediately come across the problem of not having enough "space" (not sure the word) in the message to send A and B. But if I don't really need A and B at 10 Hz and say 2 Hz will work, can I convert my 10 Hz message into five 2 Hz messages?


You can do this but now you have the problem of adding some sort of markers or identifiers to the data so you can synchronise both ends. This will take up extra bandwidth.