Hi again, Debbie
I have had a look around my academic texts; there is nothing that gives a definitive list of do's and dont's for trio sonatas. I am waiting for a call back from a colleague who, I think, had to compose a trio sonata in baroque style as part of his degree course at Manchester uni.
In the meantime, I have a score of a movement from a trio sonata by Corelli, composed in 1689. The movement is scored for violin 1, violin 2 and continuo (cello and keyboard).
I can see that where one of the violins are providing a dissonant 7th this dissonance is prepared by being sounded in the previous chord.
I can also see several examples of the 7th missing from the violin parts. The cello will not be providing it because it doubles the continuo bass line, so the keyboard continuo must be providing it.
I can also see 1 example of an unprepared 7th being approached by step - standard practice in 2 part counterpoint but not in 17th century 4 part harmony. I would try to avoid this for now.
To sum up:
* Prepare the seventh by sounding it in the same instrument, at the same pitch in the previous chord if you are using it in one of the 'solo' parts.
* The seventh demanded by the figured bass doe not necessarily have to be in the 'solo' parts. It may be provided by the keyboard continuo, so that answers the question you posted.
I will try to confirm all this later.
If you want me to run my eye over your efforts, scan them into your computer and get my email address from my website www.hopwood3.freeserve.co.uk and email me the file. I will look at it with pleasure.
Steve