OK to leave both trunks. I'd also be inclined to let it grow as a shrub, and allow a few of the suckers to develop into new stems. That'll give it a denser appearance, better looking.

Resin

I'd agree with pineresin but with one caveat--if the suckers are in fact lilac, I'd also let them grow to fill in the bush. But sometimes, lilac is grafted on privet which may also sucker. So first ascertain you're dealing with lilac suckers before deciding to keep them.

that is a very nice site.. for a very old lady ... and i mean that old shrub ...

IF IT WERE ME ....

next time you see 3 or 4 suckers.. i would just cut the big ones down to about 3 inches from the soil..

with no insult to the monster root system .... those little suckers.. will be 3 feet tall by fall ... most often such pruning is done right after it flowers.. may as well enjoy the show ...

a lilac can take that big cutting.. the proper term is: rejuvenation pruning of shrubs .. and there is a link below .... wherein you remove 1/3 per year for 3 years ...

if you want a 3 year project ... do one large branch per season ...

many times.. i have just done them all .. lilac is not a foo foo plant .. it can take anything you give it.. including simply running it over with a truck .. and then just grow like the weed it is ...

dont torture it with binding .... or anything like that.. just take a real deep breath and cut it down...

as the snake in jungle book said.. trust meeeeee

good luck
ken

Here is a link that might be useful: rejuvenation pruning of shrubs

I'm surprised lilac does so well in 8b! I didn't even know they grew that far south.

"I'm surprised lilac does so well in 8b! I didn't even know they grew that far south"

From the poster's 'My Page', it's Canada, which for 8b means the Vancouver / Victoria area. So a long way north of Wake Co, NC ;-)

Resin

Yes, Resin is correct. I'm in Vancouver, BC. Lots of lilac trees around here.

Lilacs can take lots of abuse--er rather--pruning. Nearby there was a beautiful row of a dozen bushes or so, 12-15 feet tall and wide which caught fire when burning weeds in the spring a few years ago. Burned them to the ground, just mounds where all the leaves and dirt had accumulated over the years (the house about 30 feet away was saved, although the siding suffered severe heat damage and was replaced). They sprung up from the ground, and within 2 years there was a beautiful 4 foot row again, covered in bloom.

Pruning has the same effect. Prune a branch off, and new growth will spring out below. To control size, I just remove the tallest/biggest branches near the ground periodically, leaving all the low growth and suckers, and removing the outer suckers to keep it from getting wider than I want it.