XPS 8700, two memory sticks not working

You neglected to disclose the model for your XPS.  Maybe, you can check the XPS Wiki and learn of the memory support for your system  https://www.dell.com/community/en/conversations/xps-desktops/faq-xps-desktops/647f9cb3f4ccf8a8de0df18f

Without model info it can't be further understood what is required vs. what you have.

You can possibly see the info for which PC's the RAM would work with at the point of purchase (or their support).

Alternatively, you could go to https://www.crucial.com/store/advisor and enter Dell, the line (XPS for here if you have an XPS) and the specific Model. Check the RAM type and compare it to what you bought.

Xps 8700 i tried 2gb sticks dimm seems like slot is in wrong place                                                                                             

Were the two DIMM memory sticks specifications DDR3, unbuffered, non-ECC, no higher than 1600MHz?

Did you test the two memory sticks in the white memory slots, page 13?