Every vaccine administered by doctors has specific timing criteria between it and a follow up dose (legally), as well as a time required between different vaccines (also legally). Some can be administered in close proximity, some cannot. This is based on the clinical trials that demonstrated their efficacy and the impact on their effectiveness. They don't do them all at once.
Here's a link to a lot of the more common vaccines out there. You can also find the clinical trial data with a few more clicks: https://www.cdc.gov/vaccines/hcp/acip-recs/index.html