ans = 
You cannot find ALL the points between any pair, in any number of dimensions. At best, you can describe the set of all such points, typcially as a line.
Two points define a line, in any number of dimensions. Actually, a line segment. If you extend that segment out, then it becomes a line. NO, two points cannot define a plane. There would be infinitely many planes that pass through two points.
The simplest way to define the line segment is in a parametric form. This allows the points to be vertically related, and still create a line. Thus, I would do this:
% pick two random points
xyz1 = randn(1,3)
xyz2 = randn(1,3)
Now we can define the line segment as a linear combination of those two points. What I would call a convex combination.
lineseg = @(t) xyz1.*(1-t) + xyz2.*t;
lineseg is a function handle, that for any value of the parameter t, returns a point along the line connecting the points. If t lies in the interval [0,1], then the point returned will be between the two. When t==1.2, the point will be exactly in the middle. And when t==0 or t==1, you get the corresponding point. For example...
lineseg(0)
lineseg(1)
lineseg(0.5)
When t is outside of the interval [0,1], you get an extrapolated pooint along that line.
lineseg(2) % extrapolating
And, if t is a (column vector) then you will get a set of points long that line.
xyz = lineseg(linspace(-1,2,100)')
plot3(xyz(:,1),xyz(:,2),xyz(:,3),'-r')
hold on
plot3([xyz1(1),xyz2(1)],[xyz1(2),xyz2(2)],[xyz1(3),xyz2(3)],'go')
box on
grid on
As you should see, nothing special as needed. No line fit. And again, hoping to fit a surface or a plane to two points is a meaningless endeaver, since there are infinitely many planes you could form.
