The first time the bread and wine are placed on the altar, “both hands” are specified. But I have not found anything specifying this for later in the Mass.
From the 2002 General Instruction of the Roman Missal (GIRM) approved for the USA, which can be accessed from
romanrite.com/girm.html :
“141. At the altar the priest accepts the paten with the bread. With
both hands he holds it slightly raised above the altar and says quietly,
Benedictus es, Domine (Blessed are you, Lord). Then he places the paten with the bread on the corporal.
142. After this, as the minister presents the cruets, the priest stands at the side of the altar and pours wine and a little water into the chalice, saying quietly, Per huius aquae (By the mystery of this water). He returns to the middle of the altar, takes the chalice with
both hands, raises it a little, and says quietly,
Benedictus es, Domine (Blessed are you, Lord). Then he places the chalice on the corporal and covers it with a pall, as appropriate.”
Reverence is emphasised, particularly in 2002 GIRM n. 3. I agree that using both hands would normally be a better sign of outward reverence.
Perhaps this is why both hands are not specified each time: Suppose the host being shown is a small one. If both hands are specified, this will look odd. The host will barely be visible, concealing the host, rather than showing it.