Removes all tips that do not occur in both `tree1`

and `tree2`

, then calculates
the status of the remaining quartets.

`PairSharedQuartetStatus(tree1, tree2)`

- tree1, tree2
Trees of class

`phylo`

to compare.

Returns a named array of six integers corresponding to the quantities of Estabrook *et al*. (1985):

- N
The total number of quartet

*statements*for two trees of*n*leaves, i.e. 2*Q*.- Q
The total number of quartets for

*n*leaves.- s
The number of quartets that are resolved identically in both trees.

- d
The number of quartets that are resolved differently in each tree.

- r1
The number of quartets that are resolved in tree 1, but not in tree 2.

- r2
The number of quartets that are resolved in tree 2, but not in tree 1.

- u
The number of quartets that are unresolved in both trees.

Other element-by-element comparisons:
`CompareQuartetsMulti()`

,
`CompareQuartets()`

,
`CompareSplits()`

,
`QuartetState()`

,
`SharedQuartetStatus()`

,
`SplitStatus()`