Dilbert's mom told him, "When paralleling capacitors, just add the values." As she left the room he couldn't quite hear her mutter under her breath, "Unless you grow up to be an audiophile."

It turns out this simple rule has exceptions. The usual formulas don't take losses into account. If losses are low, it doesn't matter. If losses are high, the results might not be what you expect. It's almost never an issue with film caps, but electrolytics operating at audio frequencies often have high losses. Not necessarily high absolute impedance, a myth, but high losses in terms of resistance vs reactance.

The standard measurement frequency for electrolytics is usually 120 Hz, sometimes 1 kHz for small ones, and these days 100 kHz to get ESR with switching supply filter caps, but rarely do they give you any numbers between 120 Hz and 20 kHz. It's not surprising because those measurements can be difficult to make due to the very low impedances involved, and don't show the parts in their best light.

As you combine capacitors, particularly when you bypass an electrolytic with a smaller film, there are some surprises. The first is that gaining any improvement in high frequency performance is a lot harder than you might think. If you can make measurements for the parts, the linked spreadsheet will give you the answers.

The second surprise is that paralleling a big high loss electrolytic and a low loss film cap can result in a lower capacitance than the electrolytic alone! That's completely contrary to what you might think, but it's real and verified by bench measurements. You add more capacitance and the total goes down. Since nobody has noted it before, I'm naming it The Hoffman Effect! It's not the result of inductance or resonance per se, just high loss, and is really a trick of the math for the series capacitor model. Still, it's rather unexpected. It occurs when the dissipation factor of the high loss part exceeds 1, common with large electrolytics at higher audio frequencies, and the dissipation factor of the paralleled cap is low. Skeptical at first, I had the result checked by two different EE friends of impeccable pedigree, and they confirmed the phenomena to be quite real.