You haven't actually finished a sieving reservation (and I'd have had the first one completed by now, 5P of GFN16 isn't very large) and you already want more simultaneous reservations? Here's the way it works automatically: For every reservation after you finish and upload a factor file, when I OK it your maximum reservation size increases by 1 and so does the number of simultaneous reservations. I can bump up both of those values, but I'd like to see some results first. The normal maximum number of simultaneous reservations tops out at 10. The maximum reservation size is 200P for the active GFN projects.
There's an optional device number you can append to the command line. D0 for the first GPU, D1 for the 2nd, etc. I have no idea which GPU will be 0 and 1 but you can certainly experiment. I would also use the optional block size and set it to B9 or above (B7 is default). B9 or B10 are good compromise values: lower is significantly slower, higher causes more video lag without being that much faster. You can run as many copies of the sieving program simultaneously as you want to and they can even all run in the same directory. Just don't try to sieve the same exact range twice, as both processes will try to write to the same files and at best just duplicate the work.
Sorry if I'm sounding nasty about this, but people make reservations all the time and then don't follow through. I've currently got reservations from two different people that are over two weeks old. One is for 1P of GFN16, the other for 5P. Those users haven't acknowledged my PMs and when those PMs reach ten days old, those users will just get completely cut off from the manual sieving system. It's manual on my end as well and since users can cancel their own reservations there shouldn't be any reason why I need to wait 24 days for someone to do 40 minutes of sieving. Yet it happens constantly.
So as soon as you have a track record of work getting done, please feel free to send me a PM with any requests you might have about max reservation size or number of concurrent reservations. All I ask is that you sieve what you reserve.