Well you're always out in pairs so you always have an extra on hand so to speak but waiting for support just because there is a third or fourth person that need to be nicked is a pain. Essentially you call for support and plan ahead when possible but it's annoying when that isn't possible...**** happens.
Would having 2 each be practical? I don't know how inconvenient they are to carry. I would be surprised if they are so inconvenient to carry that routinely carrying 2 sets would be impractical, but I could be wrong.
Are they expensive enough and fragile enough for the cost of doubling the amount to be prohibitive? I'd be surprised if they were.
Also, I've seen coppers out on their own. Very rarely, though. Only twice, come to think of it. Once when I was breaking into a flat(*) and a copper came by to question me. Although maybe they'd been in a car with another copper who stayed in the car. My attention was on them at the time. "It's not what it looks like. Well it sort of is, but it isn't". Once when trouble kicked off in the street outside my workplace with a member of one group threatening to stab someone in another group to death. The first car arrived in a couple of minutes with just one copper in it, who went off after the group that had fled. The second car with a pair in arrived about 20s later and followed the first. The van with a bunch of armed police arrived a minute or two later.
* I was living there and had left my keys at work, so I was trying to get in without keys and without doing any damage. But of course they didn't know that in advance, so they investigated.