you shouldn't log in twice. you're effectively halving your hd cache when you're trying to upload on two different ftp sessions at the same time, also slowing your upload performance because the ftp server will alternate between both sessions, writing one piece from session 1, sending the acknowledgement for the write on session 1, then start writing one peice from session 2 while it puts session 1 on hold, sends the acknowledgement for session 2, switches back over to session 1. so not only will your downloads take approximately 2 times as long, but you're also limiting how much your comp is able to send at once.
if you only get 192kbps upload even with only one user logged in, then your problem probably has something to do with the ftp client you're using(or could be a problem with the router or switch that you're using, as well, assuming you're using one). my suggestion, get a 3 foot crossover cable, switch your NIC into full duplex 100baset(or tx), and try it then. on my WD 40gb drive, I can get about 6 or 7 MBps upload speed (12 MBps being the cap for 100baset) which mostly has to do with the fact that the drive is only on a ata33 bus, it's only got a 2MB cache, and I've got a crappy d-link nic with very little in the way of cache itself.