val source1: Source[T,Mat] = _
val source2: Source[T,akka.NotUsed] = _
val concatSource: Source[T, Mat] = Source.combine(source1, source2)(Concat[T]())
//most common Keep.left case
source1.concat(source2)
//more general
source1.concatMat(source2)(Keep.both) //or whatever strategy