Hi Andy joining splines is that in 2018 or 2019?
Since 2015. Though in 2015 / 2016 for me. beziers worked ok, but splines were a bit flaky. You probably would not have noticed that it could work though. As I say it has limitations. The tool is the normal join poly tool.
For Splines. The new joined spline cannot have a knot (kink). So the two ends to be joined have to be perfectly in line, otherwise it will get converted to polyline. Unless one is really careful when drawing the two splines. then one often has to play around with the 'second' nodes after the splines are snapped together. it helps to draw a line tangent to the ends, and in edit mode, snap the second nodes (the green one if using 'by fit points' to the line. using 'show frames' with spline by control points in spline properties can also help.
To be honest, its often quicker and usually neater just to snap a new spline with control points to the frame points, than mess around trying to get the frames aligned.
Beziers on the other hand can have 'kinks', so Bezier works quite well. and what also works quite well. is, in properties, to change the spline curve to Bezier. join poly, change back to spline. The result is often pretty accurate. (though not always). Note - after changing to Bezier it will probably look a bit odd, that is normal, it will reset when changed back to spline. Obviously the point at which the two splines joined, will become curved. but this would happen even if one drew a new spline.