Disc of the sun is EV33.6 (from my Venus thread), i.e. 19 stops more than Sunny-16; you can maybe do your exposure calculations from that.

2 opposing filters will obviously give a fair amount of attenuation but not (I think) predictably. Most of them don't cut UV very well, and with a direct image of the sun I suspect that UV light is going to have a big impact on your exposure once you've cut out a lot of visible light.