When can i use the "Use shared PLL(s)" option in ALTLVDS

Someone else might be able to provide more details, but first off I would throw the megafunctions you want into a design. Quartus II can merge PLLs across blocks, and so I've had it do that without really diving into the details and moved along. If you get a no-fit because there aren't enough PLLs, then dig into it. (And if it does fit, it might be worth looking at the PLLs in the Technology Map Viewer, just to see what feeds what.)

Also, when creating the LVDS functions, you have the option to put the PLLs external. This is useful if you want to hand-modify the PLL in some way, like adding another output. This is more painful, and I recommend creating the megafunction with the PLL first, since it's much easier and great when it works, which is most of the time.