If there's no option to download multiple files simultaneously, I suspect there is not much more to do, Is there any convenient way I can run two instances of Newsbin simultaneously?
This is how usenet works:
1 - A typical 50 meg RAR files is divided up into 67 blocks of data and these 67 blocks are then posted to the group.
2 - To make a 4 Gb file, you need to post 80 50 meg rar files divided into 67 blocks per file (not including the PAR Files). So a total if 5360 blocks of data are posted to the news server for a single 4 GB set of files.
3 - Newsbin downloads blocks of the files. If you have 10 connections enabled, Newsbin downloads 10 blocks of the file at a time.
The connections are fully utilized, the whole time pulling data. That's how people can get close to 1 Gbps download speed. This is more efficient than downloading files in parallel because all the connections remain active all the time.
Your speed problem is external to how Newsbin downloads. Many people download with Newsbin at easily 3 times your current download speed. Running 2 instances of Newsbin won't fix your problem. It'll just add more problems.
In your shoes, I'd experiment with different news servers. I'd make sure they are all the same priority so they were all used at the same time. I wouldn't use a crazy number of connections. Maybe 8-10 per server. I'd make sure you're not running out of connections (errors in the logs).
I'd remove any security software, reboot and try some downloads. This has often improved performance for many. The only security software that doesn't seem to impact performance is the built in MS stuff.
You may find you're simply ham-strung by your wireless connection but who knows?