I'm going to do the Seattle to Portland ride this year as a 2 day 203 mile ride. I haven't done back to back centuries in 30 years (I'm 62 now) but have done 3-4 day touring rides where the 2 day mileage was pretty close to that, and here is what has worked for me in the past:
I'm normally getting in one long ride (50 - 65 miles) on the weekends and 1-2 shorter (20-30 miles) during the week after work or at lunch, when all the weather/work/family gods cooperate - I keep doing that. I try to make the longer rides be hillier every other week - that increases time in the saddle and I think hills are where I make my fitness gains. I do 90% solo rides, and the occasional group ride and I can do 60 mile rides with 2,000 feet of climbing or 4,000+ feet of climbing around here.
Then, I have two opportunities to do 75 and 85 mile rides to friend's houses and whenever I do one of those, I try to do another ride the next day. That seems to help.
I've tried several times to do one of those Century training plans, but I've never been able to stick to it. I now do a lot of Zwift riding over the winter and on rainy days during the summer - throwing in some high intensity intervals on there also seems to help.
Not trying to claim I'm fast - this past fall I did the completely flat Salisbury MD centruy in just under 6 hours and that was my fastest time for a century ever. But, I've always finished the second day before they sent out the search and rescue teams!