How far apart would you plant trees so they form an arch when they're big?

I'd like to plant 2 trees at the 'start' of my back garden so that when they're a few years old and have big outstretched branches they form something of a natural archway. I was tempted to do a MSPaint drawing to illustrate my meaning but I'll spare you the pain.
How far apart do you think the trees should be planted to achieve this then?

The type of tree is probably important, and they are both pear trees. I grew them from cuttings and they're about a foot high.

 

Pears are very slow growing HBK - hence the old addage Plant pears for your heirs.
I think you might wait a few years to get them up and over an arch - but maybe somebody knows different.
As for other trees - well I had 2 lilacs about 6 feet apart which covered the arch between beautifully.
Laburnum can do a good job - but maybe you would need to plant about 12 feet apart.
We had Rowans making an entrance arch ( to keep the witches away) but the trees were about 20 feet apart.
I think it all depends what kind of tree you want (the branches do get very big quite quickly).
Any chance of a pic ?

 

Labuburnum would be great like Alice said or two wisteria either side , pears take ages.

Have you thought of planting for now while the pears are growing have the best of both then, I would of personaly bought the pears as standard trees as this would of been easier to do plus you could ask the supplier for a variety that can reach the dizzy height that you want somthing on root stock M25

Spruce

 

It's embarrassing how long it's taken my to reply to this thread, it's been a busy week. Both thanks and apologies to everybody who replied to me.

I'm terrible with estimating measurements but I'd say my garden is about 14 feet wide, so planting 2 trees 12 feet apart wouldn't so much make an arch as it would be me planting 2 trees at each side of the garden. I did google pear tree varieties but there were so many and I couldn't find any identical.

I didn't realise pear trees grew slowly, I thought they just took a long time to germinate.

Here's an old picture taken in Autumn, it's the only one I have at hand. I'd take one now but it's pouring down outside. It might help you with both the garden width and pear tree type.

Well I didn't buy the tree I took cuttings from my existing one.
I'm considering buying some 'bird cherry' trees because they look good and grow cherries for the birds. Would these be a better choice for my archway and would they grow any quicker?
thanks everybody.