Repeating again from your other post with the same question 'causeI think there's definitely value to having a guide for the latter two sites:
I'd opt for the guided tour of the Colosseum, Palatine Hill, and Roman
Forum. The Palatine and Forum are complex sites that can greatly
benefit from a guide's knowledge, especially structures which can take
some imagination/explaining to visualize when there's little left of
them. :O)
This would be the coopculture tour you'd want (3 hours):
https://ecm.coopculture.it/index.php?option=com_snapp&view=event&id=12F8D4A5-40D1-0BE5-598B-016CF81AB14C&catalogid=F227944C-6194-7109-5694-016CFC0CD671&lang=en
It doesn't include the underground but tours which do that area don't spend much time down there, you can see down into it from above, and you'll essentially get the same info about it on a tour which doesn't include it. Anyway, the tour I've linked is a very economical option for folks counting the pennies! Note that this tour is ONLY available on Saturdays and Sundays.
Also, if I book both, would that mean doing the Colosseum twice?
Yes. You just wouldn't be doing the underground part (or 3rd ring, I believe, but won't swear to it) on the guided tour which includes the Palatine/Forum.