You say you have a 75' run. That's quite a long run.
You say that when one unit is connected to that 75' run, it works.
You say that when you "split" the signal at the end of that 75' run, neither unit works.
My take: the signal isn't strong enough to survive a split and still run two devices.
I see two options :
1) Try that switch and be satisfied to run one unit at a time it it doesn't introduce too much loss into the signal path.
2) Try a toslink amp like this :
http://www.cablewholesale.com/specs/fiber-optic-amplifier/10tr-08400.htm and see if it boosts the signal enough to survive a split.
An interesting quote from the blurb for this item :
"This optical cable booster will allow you to extend your optical cable beyond the 5 meter recommended maximum length. This device will extend an additional 20 meters."
FWIW, 25 meters is approximately 80 feet, so you're
still pushing the signal strength/length limit.