This time I post asking for help from someone who has experience with a similar setup as mine.
My intention is to move my D510MO board to a different chasis. This chasis allows to use two PCI cards horizontally, so a dual PCI riser card is needed. Dual PCI riser cards convert one PCI slot to two available PCI slots. It should be flexible, because there is a gap between the PCI slot and the PCI card.
I guess it is very likely that (almost) none of the dual riser cards in the market work with Intel boards with both PCI slots populated, so I hope that someone with a similar setup share the experience with us, in order to find a riser card which works.