Twinning

Twinning (making a twin of) has at least five meanings:

In biology and agriculture, producing two offspring (i.e., twins) at a time, or having a tendency to do so
City and town twinning
Widening of a road by construction of another one next to it, with a median in between (Canada)
Crystal twinning in crystallography refers to intergrown crystal forms which display a twin boundary: Crystal habit
"Twinning" is a jargon term in motion pictures for various special effects that allow to double (or multiply) the presence of the same actor or actress on screen.