JAP,
Yes, there is no doubt that recieving under both species is a fuller sign.
But what, exactly does the Church mean when it calls something a “sign”
Aquinas referred to a sign as an element that points towards a greater reality than itself.
We see this ourselves in everyday signs.
For example, when we look on a pole at a sign that says “Main St.”, the sign clearly isn’t a street, but rather it points towards\identifies a greater reality, Main St.
The reception of both species points towards a greater reality, the Eucharistic Banquet of Heaven.
But when then, is the PURPOSE of a sign. Well in the case of the sign for Main St. it lets us know that we have arrived at, or are on, Main St. In other words, it removes doubt.
The sign of both species removes doubt about the nature of the Eucharistic Banquet, that we Eternally recieves the Body Blood, Soul and Divinity of Christ.
But the sign has little value where there is no doubt. If I am sure I am on Main St. I have no need to check for signs. In fact, constantly checking for Main St. signs when I have no doubt that I am ON Main St. is a distraction from my travels along Main St.
Likewise, if I am sure that my personal reception of the Body, Blood, Soul and Divinity of Christ is a participation in the Eucharistic Banquet, if I have no doubt about it’s nature, then I have no need of fuller signs.
I have already recieved the greater reality to which the sign points.