1) Yes channels 1 and 6 would be good choises, transmissions at each channel will not interfere with each other. You should just avoid using channels too approximate to each other, any distance larger than 22 mhz is perfect. Channels 1 and 6 have a distance of 25mhz (each channel has a distance of 5mhz from an other). Still, you should try other channel pairs as well in case there is something else causing noise.
2) Actually unless you do some traffic shaping, because of the nature of wireless connections, the node with the strongest signal gains very significant priority -even absolute in case of large difference- over the others. So in your example, you shouldn't be surprised if you found User 1 utilizing 90+ % of the bandwidth, leaving just some left-overs for User 2!
If both users had the same signal strength, under the same other circumstances, the bandwidth would be shared evenly.