I'd also go the 2-tent route. It seems ridiculous to carry a heavy winter tent for the 7-8 months of the year when you don't need it.

For a lightweight 3-season tent, I'd also go with a Tarptent, either Squall 2 or Double Rainbow. Despite their name, tarptents are single-wall tents, not tarps--most have floors and netting and are fully bug-proof. Get your SO in on the decision, since you need to keep her happy. I suspect you'll both be happier each having your own door and porch with the Double Rainbow. Either of these tents will do fine for 3-season use. They are lightweight--lighter than any double-wall tent. Henry Shires' tents have sufficient ventilation that severe condensation inside is a rare event. I've had far more condensation inside a double-wall tent (Sierra Designs Clip Flashlight--big puddles on the floor due to condensation from the fly leaking through the inner tent) than with my Tarptent Squall 2. It's adequate ventilation, not the number of walls, that prevents or reduces condensation. And Tarptents are more spacious than the lighter double-wall tents; the latter reduce weight by severely reducing the inside space. You and your SO will appreciate the roominess.

IMPORTANT: Tarptents are most definitely not winter tents. They'll repel a couple inches of snow, but not a couple of feet of the heavy wet stuff we normally get in a PNW winter storm. Please read and heed TomD's post!

Since you probably won't be winter camping much for a while, I'd suggest renting (or borrowing) a 4-season tent in winter. I definitely wouldn't buy a winter tent until you've become experienced at winter camping and decide that you really like it. I tried it a few times and decided that I hated it, mostly because of our long hours of winter darkness up here north of the 45th parallel. Your Mileage, of course, May (and probably will) Vary. If you do decide that you want to go all out for winter camping, your experiences in a rented or borrowed 4-season tent will help you decide what kind of tent you want for the winter. At that point you can pick up a used winter tent that hopefully won't break your budget.