One language will always suffer when sim-comming. It's next to impossible to be fully effective in both languages for an extended period of time. You sort of have to decide which is more important, the English or the sign and skew your brain that way. Also, it's impossible to fully sign ASL and speak English. Definitely when I'm sim-comming it will happen that ASL will slip in (for example, if I say "give to" I'm not going to sign "GIVE TO," I'll sign "GIVE-TO" with the ASL inflection) but for the most part if you're sim-comming, you're going to be signing English.
Mostly when I sim-com I'm carrying on an English conversation with someone and there are deaf people around, so I let the sign slip a little, the idea being that if someone is "eavesdropping" and can see enough from my signs that the conversation is interesting, they can join in.
Another situation where I use sim-com is with deaf people who use it. I know someone who has a pretty good amount of hearing and uses his voice with hearing people but also signs at the same time, so I do both with him as well. In that case my emphasis is on the sign.
I would never teach a class in sim-com. Maybe there are people out there who can very effectively convey their message speaking and signing at the same time but I am not one of them (and I also suspect many people who think they are, are not). I would teach either entirely in English or entirely in ASL (or English sign depending on the students' needs) and have an interpreter if necessary.