- You can put multiple VMs on a harddisk. Preferably in separate directories to keep things ordered.
- If you format the SD card with a filesystem then it is no different from any other directory.
Combine these two and the answer should be an obvious "Yes, you can."
Things get more interesting if you want to use RAW access to a disk or a SD card, but I saw nothing in your question that indicated that you wanted to do that. Also, that would still yield the same answer as when you used a regular harddisk.
Which speed does the card need to run a Windows VM (for testing websites) and a Ubuntu server (for the webserver)
As fast as you want. Get a slower card and you need more patience.
Again, the same as with regular harddisks.