I'll use Bolin as an example. He is a lava bender and may be a metal bender as well. If he were, would it be possible that he could bend metal and lava at the same time? Or lava and earth? The reason I thought of this was because in Avatar:The last Airbender, it showed avatar Roku bending all four elements at once and I just wondered if you could do this with sub-skills.
I don't know. I think it's easy to learn more than one subskill in some elements than others. Bolin, for example, only knows lavabending possibly because he doesn't have the ampitude to learn metalbending. Just a theory i thought up.
At precisely the same time, probably not. But there is no reason why a sufficiently skilled bender in any discipline could not rapidly switch between one subskill and another while bending, using them in such quick succession as to be nearly simultaneous - as mentioned, Ozai using firebending to propel himself while creating a lightning bolt, during Sozin's Comet.
But Ozai stopped using the fire jets everytime he attacked. Okay, maybe not everytime, but most of his attacks he stop using them, attack, and goes back to using them(even when he used normal firebending). The most he did was three fireballs, i believe he couldn't do more than that simultaneously, only the avatar can do five and four is too close to five.