John Gill Commentary Genesis 48:6

"And thy issue, that thou begettest after them, shall be thine; they shall be called after the name of their brethren in their inheritance." — Genesis 48:6 (ASV)

And your issue, which you beget after them, shall be
yours
The children of Joseph, that either were, or would be begotten after Ephraim and Manasseh; though whether ever any were is not certain; and this is only mentioned by way of supposition, as Jarchi interprets it, "if you should beget" these should be reckoned his own, and not as Jacob's sons, but be considered as other grandchildren of Jacob's were, and not as Ephraim and Manasseh: [and] shall be called after the name of their brethren in their
inheritance ;
they should not have distinct names, or make distinct tribes, or have a distinct inheritance; but should be called either the children of Ephraim, or the children of Manasseh, and should be reckoned as belonging either to the one tribe, or the other, and have their inheritance in them, and with them, and not separate.