Can I use a pci-e x16 slot for multple cards?

What you want is a riser card like this. The problem is that almost all standard cases won't accept the the add in cards, especially if they have external interfaces, unless they were designed with them in mind. Such cases are typically 2U rack mountable. Also, with this particular card, there may be other clearance issues since the cards point back toward the CPU.

this http://www.bhphotovideo.com/c/product/547755-REG/Dulce_Systems_963_0700_0_PRO_EX_PCI_Express.html does what you specified, just in a large scale.

In the end you may find it cheaper to buy a convenient new motherboard?