Here's your hug! Mine were 15 months a part (with an older daughter at 3 1/2 years). She was always independent but the two younger ones were and are still so very close today.
Here's what you'll be dealing with.
If you bottle feed and you have expensive formula and even if the older baby is weaned, that one will want the formula so go back to the bottle so the older one won't come flying by and "stealing" it while you're burping the baby.
Second, watch the older one because he's not goning to understand soft spots, or that his hugs are too tight and kisses too hard and might be a little jealous as well.
If the older one is a climber, keep a close out with the infant in the crib. I could blink my eye and my son would be in the crib with my daughter wanting to play with his big kid toys.
The added child may be a problem going to the grocery store what with one basket full of babies and then the second cart for the groceries. That was really the only time having the three all under the age of 4, and of course the bottle bandit.
Now that's some of the downsides. Here are the upsides. They will be close once the little one gets bigger. They will go through schools together, school events, braces, cars, college and weddings all at the same time (and you think you have a tight budget now!). (If they are opposite sexes, they'll share each others friends as potential dates.)But when they spread their wings you'll be done with child rearing, you'll still be young and life will be great for the two of you...until the grandchildren come about 18 months apart.
I have a friend with 9 children all about 18-22 months apart and she's managed to be sane (if one can be sane wanting 9 children) so I know you'll be fine.
And here's another hug for the road!