The Concat method will return an object which implements IEnumerable<T> by returning an object (call it Cat) whose enumerator will attempt to use the two passed-in enumerable items (call them A and B) in sequence. If the passed-in enumerables represent sequences which will not change during the lifetime of Cat, and which can be read from without side-effects, then Cat may be used directly. Otherwise, it may be a good idea to call ToList() on Cat and use the resulting List<T> (which will represent a snapshot of the contents of A and B).
Some enumerables take a snapshot when enumeration begins, and will return data from that snapshot if the collection is modified during enumeration. If B is such an enumerable, then any change to B which occurs before Cat has reached the end of A will show up in Cat's enumeration, but changes which occur after that will not. Such semantics may likely be confusing; taking a snapshot of Cat can avoid such issues.