Hi,
National Instruments does not provide cables with two VHDCI connectors. If you want this kind of cable, you have to made it by yourself or have a look at others suppliers.
The solution I can propose you is to change the connector that is located on your home-made board : I mean, you put a connector such as you have now on your E series on your self-developped board. If you do this, you would be able to connect this board and a M series with the same cable SHC6868 as you did before (in the other sense!)
These connectors are sent by National Instruments. They have the same architecture as those VHDCI. Below is a link about these custom connectors.
http://sine.ni.com/apps/we/nioc.vp?cid=4798〈=US
Hope it will help you,
Regards
DD