[e2e] on local ethernet throughput?

[Dan HE]

Hello,

I was confused by one on the questions of the available bandwidth in a
local segment.
This question could be very fundemantal but it would be confused. 
Here is a graph that addresses a segment of ethernet topology.

 
           |A|     |B|
            |       |
--------------------------------- 10Mb
    |          |           |
   |A1|       |C|         |B1|

There are five stations sharing a segment of ethernet. every stations
have
10Mb/s interface card. A and A1 are communicating, B and B1 are
communicating.
C is an observer. 
(1) how much maximum throughput can be reached between the communicating
pairs(A-A1, B-B1) at the same time? is it about 10Mb/s for both (A-A1)
and (B-B1)? or when communicating, both pairs only can share average
half of 10Mb/s bandwidth? 
is it possible to say that adding both the average throughput of (A-A1)
and the average throughput of (B-B1)  is lager than 10Mb/s!?

(2) for the observer, how much bandwidth has been used in this segment
when the others
are communicating? because in some adaptive applications based on the
local segment adaptation, they usually addressed that they are measuring
the local available bandwidth.
but I don't know how a C can infer the available bandwidth on the link?

Thanks for your reply..

Dan