Is what you propose being done? YES! It can be done. However, there are more failures than success stories so it's best to do more research before getting into it.
Seahorses are prone to ANYTHING that is foreign to them from the situation they are bred and raised, be it other seahorses or any other fish.
The bacterial problem though probably causes more seahorse deaths that parasitic problems and that comes from keeping a tank above the recommended temperature range of 68° to 74°.
While the seahorses are normally found in waters warmer than that, those waters are continually changing.
In our tanks, the contained water is a breeding ground for bacteria like vibriosis, and, the bacteria multiply exponentially for each rising degree temperature, especially at 75° and above.
I was one who didn't maximize the odds of success by doing the recommended things but I've since learned my lesson over the last 4 1/2 yrs.
MY THOUGHTS ON SEAHORSE KEEPING
However, I have a friend who just put a pair in a tank without any specific conditions set, and he was successful for several years before they perished.
There is a thread on RC now where a hobbyist has a pair of reidi's that I raised from fry, and has the tank hooked into a sump connected to another sump that is hooked to his main tank. He has a chiller inline for the seahorse tank.
http://reefcentral.com/forums/showthread.php?t=1810342
Reverend Reefer's post starts at post #5.
Now, while bacteria situations are probably the worst, parasitic problems come a close second.
Seahorses many times fall to infestations of parasites that they have not grown up with and if a seahorse or other fish placed with it has such a parasite then there is a great chance that problems will surface.
That is why, on seahorse.org, you will find that for the BEST chances of success, only house together seahorses that come from the same breeding/raising source that have been together in the same water growing up.
There is a slow protocol for acclimating seahorses to those raised elsewhere but it is hit and miss.
Also, SOME people just luck in when mixing species or sources of species.
Unfortunately I wasn't one of those people.