So I've watched several videos on youtube about how to unclog these drain lines(none of them mention special considerations for dual unit systems plumbed into the same drain lines. I have located my interior ones in the closets with my A/C units in them. Here's the kicker. My upstairs unit is piped into the same drain line as my downstairs unit. I tried capping off my top terminator with some duct tape and it worked fine no leaks. but when I put my hose in the the downstairs line, almost immediately it began draining into the drain pan beneath my downstairs unit. I unsealed the top unit drain opening and tried to vacuum the exterior (bottom) end of the drain line with a wet vac with no success. Any tips would be greatly appreciated. I called my plumber buddy but he seems to be MIA. And this is also something I'd like to learn to resolve on my own. I've always lived in older homes with isolated window units which was a much easier fix for me lol. Completely new to central gas powered units. you can PM me on here, or if you feel like you'd rather have a quick chat give me a call at (mike) @ 985-860-2245