UPDATE - 4/18@2:39PM: As can be seen in this eBay listing, the other mystery artist is Love who originally recorded the track back in 1966 (thanks RushFanForever).
According to the Encore Records website and Twitter feed, the Side By Side Series mystery release for Record Store Day 2014 will be a 7-inch vinyl version of 7 and 7 Is - the 1966 song by Love that Rush covered for their 2004 Feedback EP. The Side By Side Series includes the same song performed by 2 mystery artists, one of which is apparently Rush as shown in this photo. It's assumed that this would be the same version that appears on Feedback. It's not clear who the other mystery artist is, although the track has been covered by several others over the years including The Ramones, Alice Cooper, and The Electric Prunes. Record Store Day 2014 is this coming Saturday, April 19th. Thanks to Gerry for the heads up.