Quoting Jess, as she explains that it's even more complex than I first thought...
>>>
þā … þā does mean "when… then"
but it's not what you see in the "the more the merrier."
That word is actually þȳ, the instrumentive case of "that" (þæt), implying utility or causing something to happen. (more people = merrier times)
<<<
@howardtayler þis is a þorn in my side.