Having a backup firearm – usually a much smaller and not visible one, on an ankle holster or something – isn’t some kind of exotic practice. It’s a way to deal with losing a firearm in a conflict or having one jam. And I don’t really see a lot of potential harm. I mean, if you trust the guy with one, I can’t see as two opens much room for trouble.

In all three significant law-enforcement-firearms-policy-setting firefights that I can think of off-the-cuff in the US, backup firearms were used by at least one party (law enforcement or otherwise).

https://en.wikipedia.org/wiki/North_Hollywood_shootout

The bank robbers had both multiple rifles and sidearms – at at least one point near the end, I recall an AK-47 suffering a stovepipe jam, taking it out of commission, as at that point, the bank robber had been sufficiently-wounded that he was unable to clear the jam; he fell back to a sidearm.

One suspect had a firearm disabled by an incoming round:

At the same approximate time, LAPD gunfire struck the Heckler & Koch rifle that Phillips was firing, rendering it inoperable with a penetration to the receiver. Phillips discarded it and rearmed himself with another assault rifle from the trunk of the sedan.[29]

https://en.wikipedia.org/wiki/1986_FBI_Miami_shootout

One of the suspects switched to a sidearm after exhausting his rifle ammunition.

FBI agents also had backups:

Two of the agents had backup .38 Special revolvers (Hanlon and Risner); both would use them at some point during the fight.

One didn’t, and lost his firearm, and was out of the fight due to that.

Just prior to ramming the Monte Carlo, Manauzzi had pulled out his service revolver and placed it on the seat in anticipation of a shootout,[12] but the force of the collision flung open his door, and according to reports, his weapon either went flying out the door or was thrown to the floor.[citation needed]

Of the ten, only one, Special Agent Manauzzi, did not fire any shots (his firearm was thrown from the car in the initial collision), while only one, Special Agent Risner, was able to emerge from the battle without a wound.

Hanlon lost his .357 Magnum service revolver during the initial collision, though he was still able to fight with his Smith & Wesson Model 36 backup weapon.

At least two of the FBI agents were shot after an agent without a backup weapon had their weapon disabled by an incoming round:

Dove’s 9 mm pistol was rendered inoperative after being hit by one of Platt’s bullets…Grogan and Dove were kneeling alongside the driver’s side of their car. Both were preoccupied with getting Dove’s weapon working and did not detect that Platt was aggressively advancing upon them. Platt rounded the rear of their car and killed Grogan with a shot to the chest, shot Hanlon in the groin area, and then killed Dove with two shots to the head.

https://www.youtube.com/watch?v=cTbp5-GhwP4

https://en.wikipedia.org/wiki/Newhall_incident incident

Shortly after Gore was killed, Alleyn and Pence arrived on the scene. Davis and Twinning immediately opened fire on them with their pistols, expending all their remaining rounds, and dove back into their own car for new weapons. In total, they had twice as many weapons as the four CHP officers they would eventually face had in their two patrol cars.[9]

One officer’s shotgun ran dry and he had to fall back to a sidearm:

Meanwhile, Alleyn emptied his Remington Model 870 shotgun at the Pontiac, firing the gun so fast he accidentally ejected a live round in the process. A single pellet from the shotgun struck Twinning in the forehead, but it caused only a minor superficial wound. After expending all his shotgun rounds, Alleyn opened fire on Davis with his Smith & Wesson Model 19 .357 Magnum revolver, but did not make any hits.

Twinning’s pistol jammed after one shot, but he grabbed another M1911 from the car and exited the driver’s side.

A random passerby jumped into the fight, started using weapons from fallen officers, and needed to switch firearms because one weapon was out.

On the other side of the cruiser, Kness picked up Alleyn’s discarded shotgun and tried to fire at Davis, but the gun was empty. As Davis opened fire on him with Frago’s pistol, Kness dropped the shotgun and returned fire with Alleyn’s service revolver.

One suspect fled, but with only one firearm and exhausted his ammunition:

At 3:25 a.m., Davis stumbled onto a camper parked near a dirt road. After exchanging gunfire with the owner, 40-year-old Daniel James Schwartz, armed with a World War II surplus Enfield revolver, Davis pistol-whipped Schwartz with his empty revolver and stole the camper. Schwartz called the police and reported the theft. Within hours the camper was spotted and pulled over by deputies from the Los Angeles County Sheriff’s Department. Without any more loaded guns at his disposal, Davis surrendered.[9]