You can easily join 2 curves by selecting them in the tree, right clicking them and clicking on merge.
If they are not visible in the tree, it might be cause they are inside an object. You can split the object and the curves will be visible in the tree now. You can then merge the solids back into the object.
If they are not visible in the tree, it might be cause they are inside an object. You can split the object and the curves will be visible in the tree now. You can then merge the solids back into the object.