You need to be very careful.
Yes two pairs can be useful, especially if going somewhere that's "busy" with our customers.
Or if you've nicked someone and are going straight back out and don't have time to retrieve cuffs off those booking them in.
HOWEVER
If you decide to buy another set of cuffs yourself, e.g those with keyholes on both sides, or those which are rigid however fold in the middle to save space. You are not going to be covered by the force if something goes wrong.
Mate of mine was told off for doing this, his skipper basically said if the DP was injured and it went to court and was found that the cuffs weren't issued then its all on the officer