Yes I would offer the second breast at every feed. It is pretty normal for most LOs to take both sides by this age. Some don't and are happy with one side but I would offer it and let her take what she will, especially if you are concerned about her size.
Is her doctor or health visitor concerned about her growth? There are several reasons why LOs drop percentile so that are not due to growth problems or lack of supply. These could be that they were born late, mum was given fluids during birth which inflated their birth weight, parents are small and so on. The position on the curve is less important than the overall trend of growth (as opposed to remaining the same or losing weight).
If she is actively feeding, I.e. Suck, suck swallow type rhythm rather than just sucking, then it is safe to say that she is hungry for the milk rather than just comfort. If you do think it is just for comfort will she take a paci/dummy?
Offering formula is a very personal decision that only you can really make. I don't think it sounds necessary for her growth, I'd certainly try double sided feeding first, but I guess the question is whether you think it is necessary for your own state of mind if you are struggling with breast feeding and want a helping hand.
Hugs. It is hard. Many people start to find BFing less problematic after the 4mo point. It does get easier