If it were me, I'd probably avoid the side-by-side set up because most of the quality SBS saddles I've seen (e.g. ADM Accessories) are very bulky and heavy.
You have lots of options for mounting - changing the finder bracket would be the easiest. The Orion Mini Autoguider package comes with a base for its finder bracket (it has two 1/4" holes predrilled), though I'm not sure what the spacing is on your ES ED80's finder bracket. Looking at the pictures for the ES ED80, it looks like the existing finder bracket can be easily removed without damage (it's just a few screws).
An alternative option is to put rings/clamshells around your ED80, another set of rings around the Orion mini guider, and then a dovetail in between the two. For example, take a look at this kit:
http://admaccessories.com/V_Series_Guidescope_Kit.htm
The 75mm rings will suit the Orion mini guider perfectly (I've done it). You just need another set of rings to go around the ED80. You could even replace the ED80's "foot" mounting with a dovetail bar attached to the clamshell rings, though it probably wouldn't be necessary.