How can I straighten and strengthen this joint?
I can remove the screws holding them together if needed, but that's pretty much all I can do.
That railing is probably fastened to posts on both to the left and right of your picture.
I have to say that whoever installed that railing in the first place with a splice like that mid span between supporting posts did not do a quality job. A splice should normally always be over a supporting post of be part of a double or triple beam made up of pieces of lumber with all splices offsetted a good distance from each other.
That should've never been mitered there. All miters should be on top of a support post...
If you're looking for a quick and inexpensive fix, cut a piece of pressure treated 2x4 about 24" long and attach it to the underside of your 2x4.
Last option, replace that piece of 2x4. Cut at center of supports on left and right and reattach your new 2x4.