Double Bubble Balloon Arch
Can I stuff an 11" clear diamond balloon with a 9" one?
I believe what you are trying to do is to create a so-called "double bubble," where a smaller solid colored balloon sits inside a larger, clear balloon.
Mostly these double bubbles are done with 16 inch balloons for the larger one, and 11 inch for the smaller one. I've uploaded the image of a double bubble arch as an example.
But you should also be able to do it with 11 and 9 inch balloons.
How to best do this...
Slip the 9 inch balloon over a chop stick, which makes it much easier to insert it into the 11 inch balloon. Make sure that the necks of the two balloons are aligned before withdrawing the chop stick.
Now inflate the outer balloon first and then the inner balloon. The trickiest bit is to knot both necks at the same time without losing too much helium (or air).