The Two Doctors isn't bad, but it isn't spectacular either - and as a follow-up of Holmes after The Caves of Androzani, its pretty underwhelming. Plus, a lot of it is really, really boring. Its incredible, but it is. I like to think that if those three episodes were edited down to 25 minutes, they'd be more entertaining. The 45 minute format hurt the show back then.
Of course, the biggest plus about it was Patrick Troughton as the Second Doctor, who had substantially more to do than he did in The Five Doctors (still not a lot, though). AND, with Frazier Hines as Jamie, no less. The first half of the first episode is really neat.
The best parts of the Two Doctors were the first 2 episodes, adding the third part made it feel padded. The beginning was exceptional.
I suspect that they kept Baker and Troughton separated so much was because Colin couldn't act his way out of a paper bag (nice guy that he is) and Troughton was phenomenal.
But, the concept of just taking a regular episode of the season and getting a classic Doctor and companion is one that is totally doable today, even more so.
And, of course, they'd need a good story. No premise, no matter how good, can survive a poor story and a poor execution. I'd also add that just being a regular episode of a season, rather than a special story, reduces the stakes and expectations.