Add Constraint to check value exists in a non-unique composite key column
As a possible answer to my own question, this could be achieved by adding a Check constraint with a function such as the following:

CREATE FUNCTION dbo.CheckSequenceKey (@SequenceKey nvarchar(10))
RETURNS bit
AS 
BEGIN
  DECLARE @retval bit

  IF EXISTS (SELECT 1 FROM dbo.Seqs S WHERE S.SequenceKey = @SequenceKey)
      SET @retval = 1
  ELSE 
      SET @retval = 0

  RETURN @retval
END;
GO

ALTER TABLE dbo.Sometbl
  ADD CONSTRAINT chkSequenceKey
  CHECK (dbo.CheckSequenceKey(SequenceKey) = 1);


EDIT:

The Check constraint prevents inserts and updates to the 'Sometbl' table where the new value for 'SequenceKey' is not found within the 'Seqs' table and this is my preference over the use of a trigger. 

However, the check constraint doesn't restrict updates to values in the 'SequenceKey' column in the 'Seqs' table or the deletion of entire rows from this table, which could lead to orphaned records in the 'Sometbl' table. Therefore I've taken the below from EzLo's answer to be applied in addition to the check contraint and function for a more complete solution:


Then for dbo.Seqs you can rollback operation that
attempts to update or delete if the SequenceKey is being
referenced:

CREATE TRIGGER dbo.utrCheckSequenceKeyReferencesOnDelete ON dbo.Seqs
    AFTER DELETE
AS
BEGIN

    IF EXISTS (
        SELECT
            'deleted records have a SequenceKey being referenced'
        FROM
            deleted AS D
        WHERE
            EXISTS (SELECT 'the sequence is being referenced' FROM dbo.Sometbl AS S WHERE S.SequenceKey = D.SequenceKey))
    BEGIN
        RAISERROR('Can''t delete SequenceKey being referenced in dbo.Sometbl', 16, 1)
        ROLLBACK
    END

END

-------------------------------------------------------------------

CREATE TRIGGER dbo.utrCheckSequenceKeyReferencesOnUpdate ON dbo.Seqs
    AFTER UPDATE
AS
BEGIN

    IF UPDATE(SequenceKey) AND EXISTS (
        SELECT
            'updated records have a SequenceKey being referenced'
        FROM
            deleted AS D
        WHERE
            EXISTS (SELECT 'the sequence is being referenced' FROM dbo.Sometbl AS S WHERE S.SequenceKey = D.SequenceKey))
    BEGIN
        RAISERROR('Can''t update SequenceKey being referenced in dbo.Sometbl', 16, 1)
        ROLLBACK
    END

END