If the big crucifix in the sanctuary is such that it would not be confused with the altar cross, then there’s no problem having a small cross on the altar.
Here’s a link to the Holy Father’s visit to Washington DC last April, which clearly shows a very large crucifix suspended behind & above the altar, and another smaller crucifix on the altar itself.
vatican.va/news_services/liturgy/photogallery/2008/15042008/index.html
If this is forbidden, someone forgot to inform the Pope.
The document
Built of Living Stones is merely a suggestion. It carries no force-of-law except when it actually quotes the liturgical laws.
There is no liturgical requirement that there be only one crucifix in the church, or that the processional cross must be hidden.
The actual liturgical law is GIRM 122
The cross adorned with a figure of Christ crucified and perhaps carried in procession may be placed next to the altar to serve as the altar cross, in which case it ought to be the only cross used; otherwise it is put away in a dignified place. In addition, the candlesticks are placed on the altar or near it. It is a praiseworthy practice that the Book of the Gospels be placed upon the altar.
What the GIRM is realy saying is that IF the processional cross is to be used as the altar crucifix, then there should not be another altar crucifix used. That’s not the same thing as saying that it should be the only crucifix in the church building.
“Put away in a dignified place” is not quite the same thing as saying that it must be “out of view of the congregation” As long as it’s placed somewhere and in such a way that it would not be confused for a 2nd altar cross, or otherwise detract from it, it can be placed in any dignified location.