Both the upgrade to valygar's katana and the Grandfather of Assassins +5, require the dagger of the star +4 as a component; does that mean i would be able to forge only 1 of them? Or eventually would i be able to have both of them?

You can forge only one of those two items in v5.

In IA v6, Valygar will have a new kit and his katana's abilities will also change to match better with the new kit. Hence, upgrading the katana won't require that particular dagger any more. The ingredients have changed (he is no longer going to backstab enemies and doesn't need the ability to go invisible with a chance per hit; the improved katana will have other sort of abilities).

But in general, there other upgrades too which have common ingredients, which means you can't forge both items in the same game. For example, there are more than one item upgrades which use the Maze Sigil or the Powdered Body of Dracolich as ingredients. You will need to choose and decide for which upgrade you prefer to use those ingredients. Making such decisions will also be part of your general strategy.