The World was all before them, where to chooseIt's beguiling stuff, but playful too. It hinges, I think, on 'solitarie', or rather on the seeming paradox of calling two people bound in lifelong companionship 'solitary'. The faint whiff of paradox, or flat contradiction, haunts the lines: how is it they apparently have (646) unbounded free choice, whilst simultaneously (647) being guided by Providence? How can they, the first creatures, have a world before them? (surely nothing comes before Adam and Eve?) But above all, of course, the emotional jarring of 'hand in hand' and 'solitary.' The point, of course, is that human is not sufficient companion for human; there's a tertium quid, because there's God. Adam and Eve have wilfully separated themselves from God, and hence their solitude. But that is not to say the Deity has abandoned them.
Thir place of rest, and Providence thir guide:
They hand in hand with wandring steps and slow,
Through Eden took thir solitarie way. [XII: 646-49]
This is what underlines the sonic logic of the lines: this pairing, the solitary Adam-and-Eve doubling, is haunted or echoed by a third thing. Take the alliteration: World and was, echoed a little along the line by where; steps and slow leads over the line break to solitarie; with wandring plays a similar game with way. The interplay of voiced thetas (The ... them/Thir ... thir/They ... with/Through ... thir) anchors the lines.
There's also something clever, though a little harder to untangle, going on with the interference pattern established by the regular iambic ictus (di-dum, di-dum, di-dum, di-dum, di-dum) and the more subtle patterning of classical long and short vowels: this pattern being approximately iambic in 646 and the first half of 647, then lengthening dactyllically for the rest.