I'd like to be able to write a tie that goes across a first and a second time ending. Please advise me, how this can be done?
The simple answer is - just write it!
Assuming the tied note starts outside the ending bars, insert the tied note before the first ending and tie it into the first ending. You'll have a Master Repeat Close at the end of the bar. Then insert the second ending and insert the tied note again. You'll see that the original note is in fact tied, separately to both of the other two notes.
Something like this where _ denotes a tie, and n a note.
n_ | 1¬_n :|| 2¬_n | n
In the above, be sure that the pitches of n, 1, and 2 are exactly the same. That is, if n has an accidental, be sure that 1 and 2 have the same accidental.
Thanks - You're correct - nwc is better than I had thought.