The Replacement Dome Assembly # BVD0449-A01 that you referenced does not come with a gasket - it is the replacement lid only.
For the seal, or gasket, you will want part # BVD0455-01. view full answer...
The MORryde Rubber Equalizers for Tandem Axle Trailers - 33" Wheelbase - 8K item # MR87ZR do come as a pair and you will be able to verify the cost on the product page. view full answer...