Mammals can have twins, and naturally, oviparous creatures can have twin yolks.
Usually, it's very difficult to hatch two lives from a twin yolk egg.
The reason is that the internal space of the egg shell is limited, making it difficult to support the simultaneous development of two lives, and there is a risk of insufficient oxygen supply and compression. Moreover, unlike mammals that can replenish nutrients at any time from their mothers, once an egg is laid, it can't replenish nutrients from the outside world; the total amount of nutrition is fixed, making it hard to support two lives maturing simultaneously.
Under such difficult conditions, any successfully hatched twin yolk egg can be considered a miracle of life. As long as the newborns can come into this world healthily, being a bit frail is not a big deal at all.
