There is a nice concept called subsidiarity. Considering the fact that Belgium as a nation state has a ministry for foreign affairs as well as a ministry for economy it is fair to assume that the competences should be there. After all that is what the citizens vote for.

Whether or not the regions have a certain say on economic and foreign policy is an internal debate. In centralized states such as France this would be a no-go. In Belgium, as well as in Germany, Austria and Italy, it's the other way. But if the nation state holds the competences to negotiate trade deals, it is fair to assume that the nation state should be the only party to represent the people's interest - regarding this specific topic. After all if you want to vote against TTIP you change your national government, not the regional one, and especially not the local mayor. Most people understand this principle that certain institutions have different competences and obligations. Therefore they don't change their mayor because s/he didn't do anything to stop the war in Syria or Ukraine...

Granting two institutions the same power leads to infighting that completely goes against the idea of federalization.

Now should regions have a say on FTAs? Absolutely. They should make up their own mind (just as local and communal levels) but in the end they should form a democratic position on their level. In the case of Belgium the 3 (well technically 4) regional governments that do have competences regarding economy (which are the Flemish government, the Walloon region as well as the capital-region of Brussels) should make up their minds what this FTA could mean for their region and then lobby their national government to support their interests.

But allowing a minister of a subordinated level to hold the same power as the national minister makes the latter redundant and undermines his raison d'être.