"Looking for two novels from my youth" Topic

I have been re-reading SF novels from my teen years when I read one a week, at 50cents not too hard. They were typically the books with two stories, one starting on the front the other from the back/

Two I remember but cannot name. Maybe short stories.
1. Man arrives at an investment house in 1500s (or so) Venice with a penny to invest. He gives instructions on what to invest in. 100 years later a man shows up with instructions to invest in Spain, later a man comes to say move to Holland and so forth. In 1900 he shows up at investment firm with new instructions. In 1960 he shows up again. They know it is the same man because someone in 1900 took a picture. He is now the richest man in the world with all of his investments. Now he says put all my money into electricity as I need a great surge to power my time machine:)

2. Mid 20-th century. Russians loose a deadly virus around the world. They offer a vaccine if countries surrender and allow occupation by soviet forces. All countries do. Every month people must report to special hospitals for a booster. A man born at home discovers he does not have the disease. It turns out the reds are giving people boosters of the disease. He starts some sort of revolution. Reminds me of what COVID could have been, but introduced by CCP.

Sounds like an Ace paperback. Is there a ludying on line? Not new books of course, nor in print?
Google. Collectors might have a site.

Wikepedia has a list of Ace Doubles. Might see whether any titles are familiar.
link

Would you recognize the covers? Google the (possible) title and images, and you should get the cover.

Somehow I missed those during my science fiction paperback period!