In DFP, you would create an Order. Let's call it Order A. You create a Line Item 1.
Line Item 1 would be assigned 2 Ad Units. Let's call them Ad Unit Sponsored (50x250) and Ad Unit Image (300x250). Line Item settings would be say Sponsorship 100% Goal, Deliver Creatives: As Many As Possible (aka Roadblocking).
You add in the Creative. Say for 50x250, you create a Third Party Creative with the code:
<pid="sponsorship"> Proudly sponsored by <spanid="sponsorship_name">Google</span> </p>
For the 300x250, you put in your Image Creative.
In your website, when you generate the tags be sure to enable Single Request Mode.
So in your website, you dump the Ad Unit code where you want them to appear.
If you have multiple sets, you would create another Line Item using the same method. So each Line Item would have 2 Creatives that would sync together when displayed.
When Goal based Line Items have a % value greater than 100%, then the value becomes relative. This makes it very easy such that you can have multiple Line Items all with 100% Goal and they will basically do an even split between all of them.
This method basically puts everything inside DFP which may or may not be what you want.
An alternative, like I mentioned, is to put the entire Creative, sponsorship text and image, into a Third Party Creative type. You would need to host the image yourself though. This method is probably easier to do as you don't have to mess with Roadblocking.