Okay here's the scoop as I see it:
On both supplies, connect the + sense to the + out, and the - sense to the - out.
Connect the + of one supply to the - of the other supply. This is the center tap.
The Grounds are almost certainly chassis grounds. These can be checked by simply ohm-testing them to the case.
Assuming they are to the case, you have a choice. You can either connect them to the center "0V" point, or leave them floating. Remember, if you geound them at the center tap "0V" point and then feed other units which have any connection to ground, you're setting up a loop. -If you're powering a unit that has no connection to electrical ground, this is a way to do it.
A better way is to run seperate ground and 0V wires. Instead of 3 wires with +18V, 0V and -18V, run four, with +18, 0V, -18 and a seperate ground. Grounding at a single, central point will allow flexibility inb things like a console installation. If you're using one set of PSUs to power half a dozen different boxed DIY projects, staring the ground and the 0V at the PSU will work well as a single point... but take a tip and DO NOT ground 0V at the destination units, or you can have fun chasing loops down later!
How's that baby coming along Bluebird?