Context: in the Dead Tube manga, a villain is talking about how she turned people into zombies by giving them a hallucinogenic drug and explains that they are "contagious".
その薬の濃度をメチャクチャ濃くしたヤツをゾンビ達の歯や爪に たっぷりつけておいたの！ 要するに… 今後はゾンビに襲われた者はゾンビに感染する！
Could you help me parse the first sentence? I understand that the first part (その薬の濃度をメチャクチャ濃くしたヤツ) is a relative clause meaning "Those (zombies) in which the drug is extremely concentrated". What confuses me is that second を. What is the verb for this object? It has to be つけた, right? But it doesn't make much sense. Also, if ヤツ refers to zombies, why is there another reference to them (ゾンビ達) after the second を？
Here you can see the original page. Thank you for your help!