I am doing an MD and a PhD in Computer Science. My undergraduate experience was one computer science class taken in my senior year (that's what got me interested in CS) and a couple programming-heavy genetics courses (second-semester senior year). I worked one year as a research programmer afterwards before starting the MD PhD program, which gave me some practical CS experience. It's been a bit difficult catching up with some of the theoretical CS background that I need (the folks in my PhD program who studied CS in undergrad obviously started out knowing more CS than me) BUT it's entirely doable. There are SO MANY OPPORTUNITIES to combine CS with medicine. There are a ton of people in CS who really, really benefit from a medical perspective. I have been surprised by how helpful it is to have a medical background when tackling my research questions. It will let you avoid the common pitfall of designing a new method that is in fact entirely useless based on the way medicine actually works in real life. My research does focus on medical computer science projects, but if you're doing an MD and a PhD (even a computational one), it makes sense for your PhD to have some kind of medical or biological connection
The key is to choose a program that will allow you to pursue a computational PhD. I only interviewed at MD PhD programs that would give me the freedom to pursue a PhD in whatever area I wished. There are some MD PhD programs that explicitly restrict the types of PhDs you can pursue, e.g. only biological options. However, some MD PhD programs will let you pursue a PhD in whatever area you want, even if it's computer science. Just make sure you ask when you're interviewing, to ensure you end up at a program that will match your interests. I would highly encourage anyone interested in computation and MD PhD programs to choose an MD + computer science PhD. You will have more opportunities than you can possibly take advantage of, and you'll get to work with a lot of awesome people.