I drafted Decker and have had him all year. I acquired D Thomas week 5 through a trade. I have had both ever since, and start both every week. Here are their stats for each of those weeks (standard scoring):
As you can see, Peyton spreads the wealth around. With the exception of last week's fluke for Decker (Stokley saw pay dirt instead), it's like having two WR's that average 14.4 points per week each. Not shabby at all! They are ranked #7 (Thomas) and #11 (Decker) overall.
So, having both is a good proposition. And, if you can make it happen before tomorrow, you'll notice Week 8's Combined 36.0 points came against SD, who they are playing tomorrow.
Manning is having an MVP-type campaign this season. Neither are hurting for points, but by having both you (almost) ensure you will get production. I wouldn't worry too much about Manning going down. Having both receivers seems to be a better risk than having only one and assuming Manning will get hurt in the remaining 6-7 weeks of the season (depending on your playoff structure). That's my opinion.
RB: Martin / Lynch / Lacy / Ridley / Wilson / Michael
WR: A Johnson / VJax / Gordon /Ja. Jones / Nicks / Douglas
PK: Bailey; DST: KC