I use mixed environment on this and i planning to make a whole campain...
Car: Canyon
Env: Stadium
but here is the problem
It don´t show that it was ment to be played like this, not very clear anyway
maybe you can use this prefix for mixed tracks ?
Canyon-Stadium, so it show that its a mix track and need to be played like this to be able to submit ?
maybe you can came up with something smarter, dunno

http://tm.mania-exchange.com/tracks/view/39222