These projects seems to come at tax policy from very different directions. US Taxes helps you fill out the specific form needed to file your taxes in the United States. OpenFisca helps you model a country's tax policy overall.

It would certainly be interesting to connect the two, but I suspect that you'd do that after finishing US Taxes. Specifically, you could take a completed return from US Taxes and transform it into an input for an OpenFisca model of the USA tax code.

As it stands, nobody has developed a USA model on OpenFisca. Perhaps that could be a next step, pursued in parallel with the US Taxes effort.