Just prune out the portions that are against the fence. It will rapidly replace them with other suckers, hopefully in a more desirable, away-from-fence location.

" I would hate to have to cut the tree if there is an option not too..."
Are there no options to tie them together?

First, it is not a tree but a suckering shrub. It is actually recommended to prune out the older base stems periodically to encourage newer, more vigorous growth. It would be better for the shrub to prune them out in preference to tying them up.

it is not a tree .. its a shrub ..

google rejuvenation pruning of lilac ... and just remove 1/3 to 1/2 of the plant after bloom .... 3 to 6 inches from the soil ... you might be surprised.. what your plant will turn into.. in the next few years ...

here.. i did it for you ... might be some good video there also ...

https://duckduckgo.com/?q=rejuvenation+pruning+of+lilac&t=ffcm&ia=web

never trim a lilac at height ...

ken

Cut the ones touching the fence off close to the ground now. After those sprout replacement shoots, cut these off as well - to keep the shrub away from the fence. Any other major pruning you might want to do to the rest of it should be done at end of winter, cutting down to about 18 in. If you just cut back the oldest ones during occasional years, there will never be a time when the plant looks seriously whacked at. And has no flowers at all, something that will be the case for several years if the whole thing is cut low within the same period of one or two years.

Sorry for the confusion, yes it is a shrub, I just posted in the Shrubs section.
The shrubs are not mine and they give us privacy so we are reluctant to cut them down that low. It will take years to make it back to what they are today!.

You don't need to cut down the entire plant. Just remove a few of the back stems (with the owner's permission) that are leaning against the fence. You won't even notice they are gone..........

I just went to see them again and cutting that will remove the part of the plant that is gets on my property. If I cut those I will see just the sticks for while. This will be easier if I install a 6' fence (currently it is a 5' one )

Ivan, putting up a fence is an expensive solution for an easily solved problem. As others have said cut the stems which touch the fence right off at ground level. There will not be any 'sticks' to look at because you're cutting flush with the ground. New stems will grow. They won't take years. Just keep cutting out any which start growing back over the fence. Lilac actually benefits from this treatment.