They'll have to lower the road for the bottom deck. The height isn't there otherwise, esp for double deck buses.
It had two problematic narrowing at the towers too.
I thought this looking at it the other week. A double decker just fits under now, how are they going to add another level. But I suppose as you say, they must be planning to add a lower deck
More on the double decker plan here:
https://www.lbhf.gov.uk/articles/news/2021/03/study-double-decker-proposal-concludes-hammersmith-bridge-could-re-open-pedestrians-next-summer?dm_i=BGN,7AKFQ,1LO53J,TKU9I,1