It turned out to be incredibly simple for both seats to become covered. I tried out the Yepp's cover this morning by zip tying the back poles to the seat and then tucking it over the windshield. We rode around like that all day without much of a problem. I was worried that it would impair my vision of the road in front but it didn't. When I got home this evening, I secured the front pegs with some stick on outdoor velcro and it was good to go. The back poles will slide in and out easily for quick mounting and dismounting of the wee one.
The Peanut Shell's cover followed the same instructions as Everyday Adventure's. I drilled in two 1/4" holes at the top of the seat and two more along the sides of the cross bar. My grommets were too loose so I used electrical tape to secure them. I didn't want the poles in the front of the bar because my big guy already gets stuck getting in and out from under it and I didn't want them poking him in the legs. I can't wait to give them both a go tomorrow. The weather is heating up and I think this will make them both more comfortable and willing to ride longer distances in less than perfect weather. Also, if I need extra protection from the rain or sun, I now have a support to add on the stroller shades/rain covers as needed.