Hello Errodr:
One 2 x 6 added should be enough to strengthen the split one, just make sure it has bearing on both ends so it is holding the load rather than adding to the load.
In one case I worked on there were a couple of 2 x 6s split and more sagging. We added several 2 X 8s and used a jack from the floor to assist the roof sag. When we removed the jacks, the 2 X 8s held nicely.
Glenn
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