Sway control for 2nd trailer

[Subby]

Last weekend I hooked my buddy's 27ft 5th wheel behind my 03 QC SB then hooked my 27ft Wells enclosed snowmobile trailer behind that. Around 55 things seemed to wag a bit and any movement of the wheel was amplified on the 2nd trailer. Sort of scary at 65. We even tried a good HD equalizer hitch but only helped a bit. I've towed for years just never doubled up like that. I know there are sway controls for tongue pulls out there but have no experience with them. Anybody have some words of advice on how these sway controls work and brands?