I've just gotten to Child 75, "Lord Lovel", mentioned by Guest RA above. It features the rose growing from one grave, briar from the other, with the 2 forming a "lovers knot" growing up to the church steeple:

Lord Lovel he stood at his castle-gate,
A-combing his milk-white steed,
When up came Lady Nancy Belle,
To wish her lover good speed, speed, speed,
To wish her lover good speed.

‘Where are you going, Lord Lovel?’ she says,
‘Oh where are you going?’ says she;
‘I’m going away, Lady Nancy Belle,
Strange countries for to see...’

When will you be back, Lord Lovel?’ she says,
When will you be back?’ says she;
‘In a year or two, or three, at the most,
I’ll return to my Lady Nancy-cy-cy.’

He had not been gone but a year and a day,
Strange countries for to see,
When languishing thoughts came into his head,
Lady Nancy Belle he would see…

So he rode, and he rode, on his milk-white steed,
Till he reached fair London town,
And there he heard St Pancras bells,
And the people all mourning a-round…

‘Oh what is the matter?’ Lord Lovel he said,
‘Oh what is the matter?’ said he;
‘A lady is dead,’ a woman replied,
‘And they call her Lady Nancy...’

So he ordered the grave to be opened wide,
And the shroud be turn-ed down,
And there he kissed her clay-cold lips,
Till the tears came trickling down…

Lady Nancy she died, as it might be, today,
Lord Lovel he died as tomorrow;
Lady Nancy she died out of pure, pure grief,
Lord Lovel he died out of sorrow, sorrow.

Lady Nancy was laid in St. Pancras churchyard,
Lord Lovel was buried close by her;
And out of her bosom there grew a red rose,
And out of her lover’s a briar…

They grew, and they grew, on the church-steeple too,
‘Til they could grow no higher;
So there they entwined in a true-lover’s knot,
For all lovers true to admire...