It is always best if you can have a GOOD letter from a well known pathologist (a program director, department chair, pathologist with a national reputation). The reason being is that PDs can evaluate praise from these people better (perhaps they have seen letters from them before, perhaps they know them personally) and they also know that those people generally know what makes a good resident and pathologist.
However, having a weak letter from one of these people isn't really going to do you much of anything, particularly if it comes at the expense of a good letter from someone less well known. The fact that they are coming from practicing pathologists or research supervisors is a good thing - these people also know what it takes to be a good pathologist.
That being said, many dept chairs or program directors (like the ones at your med school, for example) are used to writing letters for people they haven't worked with a great deal. They take your CV, they talk to people who do know you. So these can be good letters even if you aren't buddy-buddy with them.
As for your specific situation only you know what is best. If you are concerned, you can probably talk to the PD at your med school, if they have a path program, or alternatively talk to the dean of your med school. They will likely have access to the letters, and they have a lot of experience with them, so they can let you know if they are weak or strong or whatever. You have two letters which you think are good, I wouldn't discount those.