I found on another forum where the last post in the thread indicates the HDS 455kHz SI beam is 75° and the 800 SI beam is 45°...(I have no proof of their info yet)...

If this is true you should see no blockage when mounting the 2d Xducer at the same height of the LSS...
(This could also explain "some" observations of the Lowrance SI images getting dark in the outer left and right regions of the SI image)...

Wilbur... Just mount them and see what you get....

Rickie