My Cars: 2007 Citroen Xsara Picasso 1.6 16V VTX 1982 Citroen GSA Spécial Estate - gone to a new home 1991 Citroen BX16 TGS Meteor - still out there somewhere!
Both is best, then you won't have to worry about the one you didn't do! I'm glad I opted to have both mine done, because once the second one was off, it was clear it wasn't far behind the knackered one!