I have not been able to make drag-and-drop work with Selenium 2, so I'm considering using Selenium 1 instead for drag-and-drop.
Before I dive into Selenium 1, are there known complications of having tests based on both Selenium 1 and Selenium 2 at the same time?
My $0.02:
Will it work?
Most likely.
Is it ideal?
Probably not.  It sounds similar to what used to be called "dependency hell"...
Most of the time, making something work takes a higher priority than making something ideal.
That being said, make sure to architect it in such a way that it's as clear as possible to the next guy why you are doing it, where and when it's being used and how it is done.
Working with both Selenium 1 and Selenium 2