I'm surprised this hasn't come up before now.
You didn't say, but from clues in your post (g-port, stealthdriver, OSX 10.3) I'm guessing you've been using the stealth hack found on Doug Wyatt's old webpages to get MIDI to and from the 5LX in OSX. Trouble is, that was developed on a 9600 which has modem and printer ports and he used the modem port which the stealth driver could find.
You had overruns because the gport: 1. Only has one connection and: 2. Won't run at "fast" speed.
So, you're hoping that with a Keyspan, you can get two ports working to run two interfaces. I'm surprised you didn't just try to use the Keyspan to access both of the 5LX ports.
I have also tried all of that. The problem is not with the Studio 4s (although there could be other issues with those). The problem is that the stealth driver doesn't "find" the serial ports created by the Keyspan. Why?
Because it doesn't even know what a USB bus is so it doesn't look there. Yes, that's an oversimplification, but essentially accurate. If one could overcome this, then as I said before, you could just use both 5LX ports and double your thru-put that way.
I pulled out a lot of hair trying to find a sneaky way to make this work without success. What I ended up doing is re-evaluating exactly why I wanted to make it work to begin with. In my case, it's because although I work in OS9 some 95% of the time, there are a few things you simply need to do sometimes that just don't have great OS9 software for. Some virtual instruments, audio editors, the occasional Audio Units-only-no-VST effect and so forth. I sometimes process tracks in OSX and then return them to OS9. I made that workable by simply using the MIDI I/O on my Delta 1010 to feed my 5LX. I have a number of Studio Patches programmed into the 5LX to cover what access to my MIDI hardware I really need. It's also enough to work a project in Logic (don't start, I get enough grief about still using SVP!) using virtual stuff and a few externals together. Actually, if I
had a stealth port, I would use it in OSX and be happy with it.
Yes, I know that answer sucks but…
I don't think there's any way to make any Opcode interface
fully functional in OSX. That would require something like a new OMS or updated stealth driver that would talk to the Apple OSX Audio/MIDI driver AND recognize the Keyspan-created serial ports on the USB bus to convert the MIDI to serial and back.
The original info is attached - not that it really helps any.
Sorry. I'd love to buy your stealth port though…