how do you cut and separate a segment of one polymsh

example:

one cube msh, cut it in 2 and seperate.

Try to press the Delete button? Of course duplicate the polymesh first - THEN select the parts you wanna delete on the 1st polymesh - and then delete the 'opposite' on the other polymesh

*note* Experiment with the various Selection Tools as it's different on how the meshes will react...

No, no. You select the faces you want to extract, then go to create=>poly. mesh=> extract(delete). This will create a new object with those faces and delete them from the original object. Extract(keep) will just duplicate those faces into a new object.

Sorry to be so negative about your idea, Qdin, but it just seems like a more risky way.

The great thing about XSI is that there are many different ways to get the same result, it all comes down to what you feel most comfortable doing.
To cut your polymesh you can use the knife tool (press the left bracket key), if you need a precise cut, you can constrain the cut to 15 degree increments by holding down the shift key, you can also use snapping to start the cut where you want it, if you just want to cut a couple of polys instead of the whole object, you can select the polys you want to cut and then use the knife tool and cut away.
After you cut your object and you want to separate the halves, just switch to polygon selection mode and drag a box around all the polys on the half you want to move, then right click and select Disconnect components then press the V key to translate(move) the polys where ever you want them.

also another way to split a cube down the middle the most even way is to select the edges that run horizontal and right click and select split edges (evenly) and itll make a new edgeloop, then you can select half the polies and extract them.