Vans always come with two keys in our DO on the shared walks so both posties can do parcels if one or the other gets back to the van first etc.
You're not supposed to share the driving.
When you sign the log book and do your vehicle checks it is you who is taking responsibility for the security and roadworthiness of the vehicle not your partner.
It's an offence under the road traffic act to allow members of staff to drive the vehicle without first completing a vehicle check and signing a control sheet (log book). That's every time the vehicle changes hands.
What happens if your partner runs a red light?
You will be responsible for the fine.
What happens if your partner has an accident and bends the van?
Your name is in the log book, your responsibility.
What happens if your partner sticks both of you into a tree?
You will receive no compensation because your name is on the log book and you should have been driving.
And what's the only argument for doing it, it saves time....