Upgraded to Marshmallow, now I can't merge my conversations in Hangouts
Of course, as soon as a I post this I figure out the answer.

I ahd to go into my SMS settings from within Hangouts and change the "Default SMS messaging account" to my gmail and "Send SMS from" to my carrier number.

Now there are no longer two accounts to switch between in the menu, and all my Google Chat and text messages are in one list. I also have to ability to switch over to the dialer.