For general use, you could run both of those monitors easily with a gpu half as powerful, 2D takes very little power to put up a frame at Windows default frequency. It's 3D where ppl run into power issues, namely gaming and 3D modeling etc. Your 3070 can handle the resolution no worries, games (unless forced by settings) will use just the Primary monitor, so your 3070 is good there too. 3440x1440 isn't all that much more difficult than the standard 1440p (2560x1440).
Unless you aim on playing a gpu extreme intensive game like cyberpunk 2077 with uber max ultra settings and full Ray Tracing enabled, and expecting fabulous fps, I don't see an issue.
Can't help with the desk, there's millions of different designs, and there's no way to gage your setup in your room, and space available etc.
Personally, I'd opt for a good quality office chair. 'Gaming' chairs don't do so well after several hours, they are all form, little function.