your not very clear with your questions and we are having to give you different advice with every new post.
you cannot use a single (output) lnb as there are 4 polarisations used in satellite signals to give broadcasters more space to send out channels, inside an lnb there is hardware to switch between these 4. you need as many outputs on your lnb as you do tuners on your card. there iare unicable and quattro lnbs but ignore these as nether are what you need.
Have a read here http://en.wikipedia.org/wiki/Low-noise_block_downconverter
A single tuner can change mux it is accessing unless a program of higher priority is aaccessing it
as long as you only need access to 4 muxes we can break this down into 2 choices.
1. 2 viewers max at one time (what you have said your looking for)
If you only want 2 streams you need to change your lnb to a dual lnb, 2 cables(shotgun cable) from lnb to tuner card and a dual tuner card, you can have both tuners able to access 2 channels at the same time on your chosen satellite regardless of what mux the channel is on.
2. full 4 muxes available (what you asked about in the first place)
if you want access to 4+ channels over only 4 muxes then you will need a quad lnb, 4 cables as well as a quad tuner and you can watch them on as many devices that can connect without throttling your network. or allow you to record as many channels as you want (hdd speed has to be taken into account).
I recommend that you go for 1 if you only need 2 channels at once.
If you are thinking of recording some programs and think they may overlap, or your wanting to future proof for more users (not taking into account more muxes being wanted or added by your provider) then go for option 2.