Giant Eagle & Kroger plus cards are owned/operated by the same company, as long as both cards are under the same phone number you can use a Giant Eagle card at kroger and a Kroger plus card at Giant Eagle to earn points & savings, BUT you have to go to the self check out. this has worked for a very long time unless they've caught on and stopped it.
Now I haven't tried this but a guy at work said if you have $.30 off in gas at Kroger and $.50 off at Giant Eagle and you scan both cards at the pump you'll get the $.80 off(like I said I haven't tried this so I don't know if this is true, but I know it works or use to work at the self check out)
Another thing that DOES work: my g/f and I both needed gas, we had $.40 off at Kroger, so I wanted to try something. we pulled up at the same pump but on oppisite sides, she scanned the plus card and her debit card then waited for me to do the same, both screens showed $.40 off so we filled up both getting the full $.40 fuel reward.