It may depend on layout. We have boards (TinyRex module using T-branch, OpenRex board using Fly-by) where 4 or 2 memories can be fitted. Of course, you need to keep the "un-populated stubs" as short as possible and it is not the optimum memory layout, but we did extensive testing for TinyRex and it works oki. We have done also a lot of testing on OpenRex and we are just planning to go to Environmental chamber and EMC lab, but it also looks good. If it helps you, have a look at OpenRex layout. You can download full Altium project here:
http://www.imx6rex.com/open-rex/PS: You may want to be careful about how you place the two memories which will be always fitted. For T-branch we placed them on the top, for Fly-by we placed them on the end.