(No mixer allowed either )
If there's enough separation between marks that would keep the dog on a line back on a return, yet, if the dog is making a conscious effort to run completely out of it to pick up a second bumper, then I'm in the camp of those who suggest re-enforcing HERE.
Seems that if the OP began teaching Doubles as Singles first, and then brought them both together as Doubles afterwards, that there "should've" been enough REPS for the dog to not even think about running to another mark after hearing a whistle and a verbal HERE, dunno'.
I give the dog a lot of credit-
Why pay for the same real-estate twice!