Though it should not be a general scenario, I have devised a way. Hope it will solve your problem.
Select AId,BId
INTO #tmp
from
(select Id as AId, ROW_NUMBER() over (order by Id) r from Tbl_ATemp) a
join
(select Id as BId, ROW_NUMBER() over (order by Id) r from Tbl_BTemp) b
on a.r=b.r
UPDATE Tbl_ATemp
SET B_Id = #tmp.BId
FROM Tbl_ATemp INNER JOIN #tmp ON Tbl_ATemp.Id = #tmp.AId
DROP TABLE #tmp
Cheers :) ..