How to charge for plumbing bill when two units share one drain?

[Jason L.]

A plumber recently came out to my duplex and found paper towel that clogged the toilet in unit 1. The thing is that the drain is shared  by both units, so technically the paper towel could have come from the unit behind them. The unit 1 tenant swears that she didn't do it, and to be honest I tend to believe her because she's proven to be far more responsible than the unit 2 tenant when it comes to paying her rent on time (albeit that's fairly circumstantial). The unit 2 tenant has no idea there was even an issue. Someone was clearly negligent here, however, I cannot prove responsibility and the blockage was found in unit 1.

How would you go about charging the plumber bill? All unit 1? All unit 2? Split?