Halliday & Matthiessen (2014: 562-3):
An extending nominal group complex may be reinforced by a circumstantial Adjunct of Manner such as both, jointly, separately, individually, respectively:
||| We had a wonderful piece of property in Connecticut, back up in the hills, || and my brother and I were both very interested in snakes and birds. |||
||| Ross, << expected to go to the Middle East on Tuesday, >> intends to meet separately with Israeli Prime Minister Ehud Barak and Palestinian leader Yasser Arafat. |||
||| For FY99, our requests for regular and supplemental appropriations [[[ to fund these operations, || totalling $1.9 billion and $850 million respectively,]]] were also approved. |||
These indicate how the element realised by the nominal group complex takes part in the process of the clause – either jointly or separately.
A number of common expressions like slowly but surely, last but not least, by hook or by crook belong to this general pattern.