I would say that double-sided feeding may help in your situation. It really is your choice. Tracy seems fairly flexible on the issue as baby get older. On P.116 of the first book, Secrets of the BW, she says the move to double-sided may happen when the baby reaches around 12lb. For some I guess it can come sooner and some certainly later (or even never).
For a 6 week old breastfed baby 1 poo in 2 days is ABSOLUTELY fine. This is normal and healthy and does not indicate constipation. Breastmilk has very little 'waste' compared to other milks. Some breastfed babies don't poo for a week or more. What may sound like straining is just her getting on with it - it won't be constipation as we know it. Constipation in bf babies is virtually unheard of. Certainly hold off on the water - especially if you have any concerns about your supply (it may reduce it) or weight gain. Any water you give 'fills up' baby's tummy and can reduce the desire for milk. This consequently means less demand on your milk supply. Even in very hot climates foremilk is all the hydration a baby needs.