They need at least 8 feet between them. First find out the mature size of the trees, then add 8 feet of open space between them at maturity. They'll be close enough to cross-pollinate and not block the sun. You'll be able to walk between them to get those limes.

Hi Shepherdess...welcome to GoY...Bathgate I think perhaps the question is about https://www.woodlandtrust.org.uk/visiting-woods/trees-woods-and-wildlife/british-trees/native-trees/common-lime/ as we can't grow edible limes in the UK.

which is Tilia x europaea a tree commonly known as lime in the UK. Its a huge tree, so I'm wondering what you are planning Shepherdess?

Hi, welcome from me too, the common lime, Tilia x vulgaris, is a very large tree, growing up to 120ft high with a spread of 50ft so if planted together, but without touching each other they need to be 100ft apart, of course there are other species which are a little smaller, Tilia caroliniana for example only grows to 30-40ft high, with a spread of 25ft, if it's this 1, then they would only have to be planted 50ft apart, so we really need to know which species you have, before we can give definitive advice, Derek.

Thankyou to you all for your answers, oh heck! No wonder no one else wanted them! Opposite ends of the orchard for them I think.

The upkeep is quite trying too as they are terrible suckerers. We had a row of them at our last house and it was an annual job cutting them all off which took ages .
The french make a pleasant tea from the flowers, fresh or dried..