As for your question, you can have Covid-19 and influenza at the same time, but it's relatively unlikely. The number of people who do will be a small subset of all those with Covid-19, and of those with both who die, the hypothetical number of them who die entirely from influenza and not at all from Covid-19 would be even smaller, even if it were possible to accurately determine. It's not going to be significant outside highly specific contexts.