This is an interesting theory, the topic about Surgebinding and Shardwielding have been discussed (here's the link ) but I don't think there's been a theory claiming Shardweilding as the second ability of Windrunners and Stonewards.
Here are a few qoutes from the topic.
Anyway, just check-out the topic. Here's my comment on the previous comments.
What if they cant access the full capabilities of the shard plate because they can't Shardwield? If Shardweilding is a separate magic system, then you cant just train yourself to get better. You have to be able to access that magic. Take for example the mistings, they can use Allomancy but they can't access all of it. Maybe not everyone can use Shardweilding.
You can't deliberately learn a magic system, you have to do something to be able to access it (embody an ideal, be born as an alomancer, get enough Breath, etc.) Why are they de-powered? I think it's because the Shardbearers can't Shardwield.
Anyone might be able to throw a stone, but even if you train very hard you won't be able to throw as far as a mistborn burning pewter. I think it's not just "inborn talent" that you need to use the shardplates effectively, you have to be able to Shardwield.
Here's a quote from the book that supports the theory that only Stonewards and Windrunners can Shardwield. It's the time that Dalinar saw The Day of Recreance in a vision. (I dont have my book with me so i'm using an ebook, i can't give the page numbers.)
and
Only two orders were mentioned, Stonewards and Windrunners. why? In another of his vision (the one when he was in the body of Heb) Radiants from the order of Stonewards and Windrunner came to help. Again, the two orders. This might be because they are the only one that can Shardwield and thus, can travel far very quickly but in a different manner. I do accept that another explanation might exist why it seems that only Stonewards and Windrunners can "travel" but i like Djando Je's theory.