The PXI-4110 is only a single quadrant power supply. For detailed information about this, please see the KnowledgeBase Why is the NI PXI-4110 Described as a Single Quadrant Power Supply?
As far as using the +20V isolated supply as a +/-10V supply, this should be fine. According to the user manual:
"Any terminal on the isolated channels can be connected to ground. For safety reasons, all the terminals must be <60 VDC from ground."
Let us know if you have additional questions about this.