We would like to take a day trip from Seattle to both Mount Rainier and Mount Saint Helen's. Is it doable?

Technically yes, but a very, very long day spent mostly behind the wheel.

We drove from Mt Rainier down to Mt St Helens and the drive was very slow and windy.

The road into Mt St Helens from the east, after you get off the highway, is even slower and takes about an hour, one way. You cannot continue west so you have to go back out the way you came and a long way around to get back to Seattle. It would be better if, after you left Mt Rainier, to go out to I-5 and approach it from the west.

If you look at Google Maps, you will see how windy all the roads are.

Mt Rainier from Seattle would be a great day. Throwing in Mt St Helens would be a very long day. If I had to choose one, I would choose Mt Rainier for the beauty but if you would enjoy seeing the aftermath of a blown volcano more (which was very interesting), then go with Mt St Helens.

Yes, to do both in one day you would have to drive about 400 miles and, unfortunately, much of the drive is not scenic. Mt. Rainier is one of the most popular day trips from Seattle.

HTTY

We did that trip 2 years ago. We were on our way back from Seattle to Portland. We loved seeing Mt Rainier even thou we only could go half way up. It was May and the road was still snow covered. When we got down to Mt. St. Helen, we went only a litlle way and saw the detruction from the volcano but we were pretty tired and decide to just finish our drive to Portland. We will be back this fall and hopefully spend our time at Mt St. Helen. But if I had to chose, it would be Mt Rainer.