Last year we did a "one and done" trip to WDW, and so we tried to cram everything into our 7 days, including both of the water parks. It was myself, DH, and our 13-year-old twins.
We combined a half-day at Blizzard Beach with a half-day at DHS. It worked pretty well, but we did feel rushed at Blizzard Beach. We could have easily spent another 2 or 3 hours there. We were staying at the Polynesian, and didn't get the earliest start (this was day 3 for us, and we were already dragging after our first 2 very long Disney days which included After Hours at Animal Kingdom and evening EMH at Epcot!), but a quick Uber down to Blizzard Beach got us there right at 10 am open. We could have benefited from getting there 15 minutes earlier at least! But even so, we found a place for our stuff easily, and for the first hour or so had pretty short lines. I will say that we experienced probably the longest line of our trip at Blizzard Beach - 45 minutes for a ride where you have to pick up inner tubes before you go up (Downhill Double Dipper?). That ride didn't feel worth that long of a wait (and the wait was posted at 20 minutes but was a lot longer)! We had a 2:45 pm FP+ at DHS, so my cutoff time for Blizzard Beach was 2 pm. Between our late start (it must have been 10:20-10:30 once we'd gotten towels, found a place for our stuff, sunscreened up, got our bearings and figured out which slide to head for first) and 2 pm, we were able to go on each of the slides/tube rides at least once plus some time in the wave pool and lazy river. No time for actual lunch, just some granola bars and cold drinks. Then I had to drag my kids out of the wave pool, and we all did a quick change (not great facilities there, so no actual showers or anything like that), and hopped in an Uber over to DHS. I will say it was very awkward going through security at DHS with our beach bag full of wet swimsuits - even though we had used the complimentary towels at Blizzard Beach we still had a big bundle of wet stuff. The security guy seemed really suspicious of us bringing that bag in, for some reason, and at first seemed like he wasn't going to let us bring it in. We put it straight into a locker near the entrance after security, where it spent a very hot day but was surprisingly not gross when we picked it up at the end of the day (nearly 10 pm after the Star Wars fireworks!). So from getting out of wave pool at 2pm, Uber to DHS, through security and stowing stuff in locker, we were wandering down towards Tower of Terror by about 2:45 pm. Not looking our most stylish in the photos we got that day! And I did regret not really getting to soak in the atmosphere at Blizzard Beach - it felt rushed. But, we did see it and I would have regretted not going at all vs. just having a half day there. So roughly 10 am - 2 pm BB, 3 pm - 9 pm DHS with an hour of transition time.
For Typhoon Lagoon, we did it the other way around - we went to Epcot for the first part of the day, then went to Typhoon Lagoon for the evening. We had tickets for H2O Glow Night at Typhoon Lagoon, so were able to be at Epcot until nearly 4 pm. I'd highly recommend doing the H2O Glow Night if it's offered when you're there! It was one of the best things we did (we did After Hours at Animal Kingdom, After Hours at Magic Kingdom, and H2O Glow Night, and loved them all). Our tickets let us enter Typhoon Lagoon at 6 pm, and we stayed until 11 pm. So we had 5 hours there, and because of the low crowds at the event, we were able to do everything we wanted, many of the slides over and over and over again, plus lots of time in the wave pool and lazy river. You could probably also do Typhoon Lagoon in half a day, but I think you'd definitely feel rushed. It felt like there was more to do there than at Blizzard Beach. Even if there wasn't a late night event offered, you could still try to do theme park morning then Typhoon Lagoon afternoon, something like 4 pm - 8 pm close (but you'd need to check operating hours. The week we were there, late August 2019, the water parks started closing at 6 pm after having been open until 8 pm earlier in the summer).
I do think splitting our days like that allowed us to get in everything we wanted, without having to sacrifice a whole day to water parks - every day of our trip was at least partially a theme park day with FP+ booked, to maximise our pre-booked FP+ over the week. I think we would have had a slightly more relaxing time, though, if we'd allowed for longer at Blizzard Beach - so maybe booked our first DHS FP+ at 4 or 5 pm? But you know, when you're trying to book FP+ you can't always be choosy, so there was an element of going with what we could get!
) and then when we're done we go to another park for the late afternoon/evening. 
