This function combines two `mids`

objects `x`

and `y`

into a
single `mids`

object, with the objective of increasing the number of
imputed data sets. If the number of imputations in `x`

and `y`

are
`m(x)`

and `m(y)`

, then the combined object will have
`m(x)+m(y)`

imputations.

## Details

The two `mids`

objects are required to
have the same underlying multiple imputation model and should
be fitted on the same data.