Connect two lab bench power supplies in series safely
The plus and minus terminals float, that is, have no internal connection to the earth ground.

The ground connection is normally an output. As such, no matter how you connect the plus and minus outputs together both chassis will still be connected to ground.

Normally, if you need an earth ground on your test circuit, you would connect it to one or other ground output on the power supply. Both ground outputs "should" already be connected together via the power line.

EDIT: As Glen points out, depending on the nature of the power supply there may be a maximum voltage you can float the outputs to. You should check the manuals if you have them.