Where are the code shares?
 

I'll need to fly from Houston to Montana a few time this year. Why no CO code share flights on United from Denver to Montana?

I thought that a primary purpose of the extra special relationship was to make it easy for us to fly mixed itineraries.

Just curious, why on earth would you want a code share?

The codeshares are there, just not the way you're thinking.

You can buy it all on one code from UA, say IAH-DEN-BZN, with UA flight numbers and IAH-DEN operated by CO.

There are only limited flights that are codeshares, so not every flight is codeshared. You usually only see them out of larger cities.