Boris Karloff and Bela Lugosi made many films together, but on the whole (interestingly enough) Karloff usually is the better man of the two. The real exception is "The Black Cat" (1934) where Karloff is playing the evil head of a devil cult, and Lugosi is seeking revenge on him for destroying his life. But more usual is "Black Friday", where (whatever his motive) Karloff is trying to improve brain surgery while Lugosi is a murderous thug. In "The Raven" Lugosi is a sadistic surgeon, who blackmails Karloff to assist his evil plans until Karloff finally has had enough. Rarely are they both negative characters totally. In "The Body Snatcher", Karloff does kill Lugosi, but Lugosi is trying to blackmail him.
The one exception where they are both extremely sympathetic but at cross purposes to each other is this 1936 film, which I feel has rarely had the audience acceptance of some of the other movies I have mentioned. In it Karloff's Dr. Janos Rukh is a hard driven scientific genius who has been sneered at by the "official scientific community" for his theory that a rare form of Radium is in Nigeria on a meteorite that landed centuries ago. He has finally gotten the support of a well financed expedition led by Sir Francis Stevens and his wife Lady Arabella Stevens (Walter Kingsford and Beulah Bondi), and has another scientist, a Frenchman named Dr. Felix Benet (Lugosi), Rukh's young wife Diane (Frances Drake) and a friend and protégée of the Stevenses named Ronald Drake (Frank Lawton).
Before they leave, Rukh is warned by his mother (Violet Kemble Cooper) that he is possibly seeking wisdom that he shouldn't and it may end in tragedy. He tries to dismiss this, but he is worried by what she says, his scientific standing, and whether or not he is going to get his due credit.
What he gets is a disaster. He finds the substance, but is infected by it's remarkable radioactivity. He finds that he is slowly burning up, and if he tries to touch people or animals they die. He's actually built up a friendship or understanding with Benet, who figures out a type of radioactive fighting cocktail for Rukh to use to counter the danger. But there are two things that are unbeatable here. The antidote can only last for a certain amount of time, and has to be replenished. And the radioactivity has affected Rukh's brain. He is increasingly jealous of Diane's friendship with Ronald (encouraged, unfortunately by Sir Francis and Lady Arabella), and he is equally upset that (due to his having to pretend to have died - the effects of the radioactivity are like that) Benet and several others are collecting the kudos of the wonders that "Radium X" is giving to man. Soon Rukh is on a murderous rampage that destroys many lives, ending with his own.
The film certainly picked up on science to an extent. Madame Curie had died recently from cancer she got due to work with Radium. Few fully understood the dangers of radioactivity in 1936, but some idea of it was coming out. The wave of murders by Rukh cause the newspapers to talk about a "curse" on the expedition. Of course, with the idea of a "cursed" expedition (on the continent of Africa) for a hidden treasure buried centuries ago, financed by a titled Englishman, we have entered archeology not physics or geology (paging Howard Carter and Lord Carnaevon).
On the other hand, Benet tries to settle the cause of the string of deaths, and reverts to an idea that was actually demolished in 1888 in England. During the Whitechapel Murders, Sir Charles Warren ordered the retinas of several of the dead victims to be photographed to see if the last image on the retinas was Jack the Ripper. It turned out he only got the photographs of the retinas of dead prostitutes. But the idea did not die. Jules Verne used it in his novel "The Brothers Kip" in 1899, and here Dr. Benet uses it. As this is a science fiction story, he finds the image of Rukh on the the plate, but Benet drops the plate accidentally and it shatters.
The film is good on many grounds, the most interesting that for a change Karloff and Lugosi are not unsympathetic towards each other. There is a type of tragic fatalism in this story that is missing from their other films. The other performances are good as well, in particular Ms Kemble Cooper. She is best remembered as Basil Rathbone's frightening sister (Jane Murdstone) in "David Copperfield". Here her final act is the only way to bring this tragedy to an end, and who can say it did not hurt her more than her target.

I've got a question for those who thought of this idea. Why? What made them think to make a second movie of this. If memory serves, didn't Cinderella end with "They all lived happily ever after"? I'm pretty sure it did, or it was implied. Wait a minute, if they all lived happily ever after, how could there be a sequel? Unless they were trying to make the most boring story in history, there couldn't be a sequel. I mean sure, they could have Cinderella singing about nonsense for two hours, but why? A movie that ends in "happily ever after" can not have a sequel; if it does then they don't live happily ever after! Something's got to go wrong which causes problems with the first ending! Why? That's not right, it's evil. That's my final word on the matter.