Dude, that's a great question. I'd love to get into that. Sorry I didn't think to cover that more originally.
The reason is I don't think Mobley playing the 4 is playing to his strengths at all. I think it's kind of clipping his wings. Tbf I did not consider Wood playing the 4 and Mobley the 5! But let me get into why I don't think Mobley should be a 4, then I'll get into why I don't think Wood should be either.
For Mobley, his strengths are pick-and-roll finishing, passing out of the short roll, rim protection, and defending the pick-n-roll as a big. His biggest weaknesses imo are his shooting and rebounding. His ball-handling is really very loose too. He definitely can still utilize his strengths as the 4, but having another big on the floor with him just makes the spacing weird. For example, if Wood is around the paint on O, that majorly limits his ability to utilize his diverse skills as a roll-man bc Wood is occupying that space already. On D, if Wood is defending the opposing team's big, you're taking away his abilities as a rim-protector and PnR defender. Furthermore, Mobley playing the 4 also encourages him to play towards his weaknesses: it encourages more jump shooting and ball-handling, which he could develop one day, but just isn't there yet. I think it's bad for his development because you're discouraging from further developing what he's best at, and encouraging him to depend on what he's worst at. He needs to develop those shooting and ball-handling skills *alongside* his top-tier center skills to be most effective.
Conversely, say Wood is your 4. Now you can utilize Mobley to his strengths as the 5, and let him develop his perimeter skills at his own pace, and Wood is a 37% 3-point shooter so he should space the floor great for Mobley. The problem is, to best utilize and develop Mobley, he needs lots of opportunities doing 5-man things - that rim protection, PnR defense, lots of pick and roll feeds. While he's doing that, what is Wood doing? You're basically relegating Wood to being a spot-up shooter most of the time and that's not the most effective way to utilize him - he's very talented doing 5-man things too. You're also now encouraging him to play to *his* weaknesses: on offense, he's encouraged to put the ball on the floor more, which he can do occasionally but shouldn't be one of his main tools. On defense, he's forced to defend forwards more, and that could result in him being a big minus defensively rather than being pretty neutral as he is now.
Maybe it could work if Mobley was as good a shooter as Wood, but he's not and likely won't be for a while, and I don't think you'd see better results than what you saw from Anthony Davis and Boogie Cousins in NOLA. They didn't really feel like a championship blueprint imo. The league is too different now and prioritizes floor-spacing too much.
It's just an issue of overlapping skill sets. If you only keep one guy playing inside, you're always forcing one or the other to do something that isn't their strength. If you have both guys play inside, now the spacing is super awkward, especially in today's NBA. If I had to pick one, my preference is Mobley because he has at least as good of offensive upside, but much better defensive upside.
Ofc, that's just what I think and I'm just some dude. I think the Rockets will at least consider going the twin towers route, and if they do it's gonna be super interesting to see if it works. It'd be a nice experiment. Personally, if I was Stone I'd try to flip Wood for the 3rd or 4th pick, maybe with some of our later firsts, and try to pair Mobley with either Green/Suggs. Regardless of me not being so high on Green, any of the four and you have an all-star potential player. I say if you can get 2 of the 4, get them no matter who they are.