Start by going to the pioneer website and downloading the VSX-52 manual. Connect your two floor speakers as the L/R front stereo channels. Then decide if you want a 5.1 setup with a virtual sub and virtual center (the missing speakers in your setup), or A/B speaker pairs playing all the time simultaneously (A+B). Personally I'd go for the 5.1 setup and the two missing speakers being virtualized as it gives you the most flexibility playing back various input sources.
For stereo sources, select the stereo listening mode and only the front L/R speakers will play. Or use the extended stereo listening mode for all channels playing. The stereo signal would be pumped out to all four channels in this case exactly like you want. For 5.1 sources the correct channels will be directed to the correct speakers - FL to FL, FR to FR, SBL to SBL, SBR to SBR, the center channel virtualized and split between FL/FR, and the missing sub sent to the fronts instead. If you go the A/B setup route instead you will hear everything in L/R stereo only, no rear channel separation with 5.1 input sources. I suppose if you only plan to listen to two channel sources then the A/B setup might be enough for your needs.