With two PRO-HDs and using only the standalone player with the bandwidth set explicitly to a little less than 2 Mbps, it should be stable. At that rate, you can get fairly good HD on low-motion content, e.g. sitcoms, drama, news, but you would lose many frames on sports or action movies. Fast motion should look decent at 640x480.
With the 350, you can't directly control the streaming rate; it will try to go as fast as it can. I'm concerned that the two units will "fight" each other for bandwidth and you may have some freezes and skips. I don't know of any way of predicting how well it would work. Perhaps you can buy the 350 at a store, where you can return it if it doesn't work well in your system.
If your Slingbox is behind a DVR, the remote viewers could of course watch the show at different times.
If one or both of the remote users are overseas, the situation becomes much more complex, because the Internet path may impose bandwidth limitations.