302. The practice of the deposition of relics of Saints, even those not Martyrs, under the altar to be dedicated is fittingly retained. However, care should be taken to ensure the authenticity of such relics.
My parish has relics in fitting displays, but the altar was not constructed for their deposition there. If relics were deposited there, we would not have them available for devotion.
As for the matter of multiplcation of altars …
303. In building new churches, it is preferable for a single altar to be erected, one that in the gathering of the faithful will signify the one Christ and the one Eucharist of the Church.
Side altars are not expressly forbidden. However, a display for devotion can have other carefully constructed aspects.
In already existing churches, however, when the old altar is so positioned that it makes the people’s participation difficult but cannot be moved without damage to artistic value, another fixed altar, skillfully made and properly dedicated, should be erected and the sacred rites celebrated on it alone. In order that the attention of the faithful not be distracted from the new altar, the old altar should not be decorated in any special way.
This is what happened in a lot of churches. Visibility is clearly considered part of the people’s participation. And when this is difficult (note: not impossible) a change is appropriate.