Hi! No, unfortunately there is no way to make that happen, even fedimint does not support that (that would be paying with tokens from one federation in the other).

We don't need to solve that though, because the idea is to use Lightning and handle the ecash exchange with multiple mints in the background. I hope we can get there, as I imagine an ecash future with a wallet having a multitude of smaller balances all over the place that you can easily move from a to b using Lightning.