Is it posible that two devices plugged in different PCI slots share single PCI bus
For conventional PCI, yes. In general, many slots in a conventional PCI system will share the same bus number. This is because all devices on the same bus segment share the same bus number. Conventional PCI is a shared parallel bus, so multiple PCI slots are simply connected in parallel (well, mostly in parallel) to form a bus segment, and all devices on a given bus segment share a bus number and are differentiated by device number. 

For PCI express, no. All links in PCI express are serial point-to-point links, so devices cannot share bus numbers unless they are physically on the same end point. Therefore, PCIe cards will in general be assigned a unique bus number and a device number of zero. Device numbers other then zero are only used in a few cases: root complex integrated endpoints, switch downstream ports, and devices that have ARI enabled to merge the device number into the function number so they can use up to 256 functions instead of 8.