I think in scenario you describe, the exchange is doomed to be a total failure. For it to be an exchange (not a teaching class) each party has to express something in its target language, which the other party commands, so that this party can help him/her addressing any shortcomings or making improvements. I wouldn't recommend working with a bridge language as a base case. Even though it may be used to solve some difficult issues.
Another point about a good exchange is that both parties have to have a good grammar and orthography level in its native language, or at least be able to find out about them when required - This is not always the case.