Cheap but risky option: Drill the crack tips. Weld the original cracks all the way along the sides to the top. Grind smooth on edges and weld in an aluminum stiffener. Monitor.
The problem with aluminum is that it fatigues and cracks at a relatively low number of stress cycles compared to other metals. And there is a weakened zone around the crack that is also susceptible to cracking given more cycles. So, the metal around that crack is not good. Welding will actually weaken the metal but will relieve the stresses in the metal around the crack. Hence you need to add fresh aluminum to spread the stress loading around the original crack.
But you have to consider that those rails probably have more fatigued zones that are getting ready to crack next. So, you might successfully patch these cracks only to move the stress adjacent to the reinforced/repaired area...and more cracks start there.
So, by repairing, you might just be moving the problem a few inches down the rail.
This is why they junk old aircraft airframes...aluminum fatigues fairly easily and quickly...and if one area fails...more are close.
Not what you want to hear...but you should try to find some low mile, used rails and replace both. If a rail breaks, you could cause way more, expensive damage or, worse, hurt yourself or someone else.