you have to learn difference between Mbps and MB/s coz it's not the same....
you wrote that you have 10 megs line... is it 10 Mbps (10 megaBITS per second) or 10 MB/s (10 megaBYTES per second) (I doubt it's 2nd one, so lets say it is 10 Mbps)?
on 10 Mbps (10 megaBITS per second) line, you can't download more then 1.25 MB/s (1.25 megaBYTES per second).... so, if your PC-A is downloading 1.25 MB/s, PC-B doesn't have any more bandwith to use.... and when PC-B starts downloading at 300 kb/s (kiloBYTES per second), PC-A slows down a bit to cca 1 MB/s (but sometimes it shows more coz it's not showing actual speed in realtime)
solution for this? on PC-A, make your alt.binz run at 50% of the actual speed of your line (in setup, put your actual speed, close it, and on alt.binz click on "No Limit" and put it on 50%) and do the same on PC-B.... both running at 50% gives you 100% of you bandwith