If two doctors you are seeing are at different addresses, they are less likely to readily share patient charts.
A lot of small, independent practices have merged into larger healthcare organizations over the past decade. These practices may still have their original, separate database from the larger system. Sometimes they negotiate to keep their system separately, sometimes they plan to join the larger patient database - but just haven't yet.
If two doctors you are seeing have different specialties, they are less likely to readily share patient charts.
Some specialists prefer to use custom software that is tailored towards their specialty's needs. Depending on cost, policies, and organizational politics, they may use a separate software with a separate database.
This was very common in the early days of medical software, but is becoming less common as organizations recognize the value of a shared patient database.
3. Patient Portal
If you've already been to two doctors, and they have different patient portals (different urls, different credentials to log in), then they do not readily share data.
Wish you could know beforehand?
Call and ask!
Try something like:
Before I scheduled an appointment, I wanted to confirm that [new doctor] is using the same system as [existing doctor]. My current doctor works out of [hospital or clinic] and their patient portal is at [url of patient portal]. Will [new doctor] use the same patient portal?
While this question is patient portal focused, it gives you a lot of insight into how easy (or hard) it will be for your two doctors to share information and communicate.