It's your choice whether you freeze the dough or the baked product - it's more a matter of freezer space than anything else.
It might be easier to bake your 2-loaf recipe and freeze the extra loaf. You're not storing it a long time, so, if well wrapped, it should be fine. I don't think it would dry out much if at all and anyway, since the extra loaf is intended for stuffing, you're probably going to want to dry it out a little anyway so the bread cubes can absorb all the good flavoring from your stuffing recipe.
BTW, did you know you can freeze biga? I do it all the time. I make a big batch and when it's done its thing, I divide it into suitable portions, wrap it well in plastic wrap and pop it in the freezer. The day before you're baking, take it out and let it defrost in the 'frig. Let it come to room temp (takes about an hour) right before you use it.