I'm a SAHM of two sons as well--and my DS was 17.5 months when my second DS was born, and he did understand to some degree about the new baby. It made a lot more sense after the birth, of course (which was at home, as planned, and he watched and even said "Baby... Momma?" while pointing to the baby and then to my stomach at the time they were first introduced--so he DID get it a bit.). The biggest problem I had was my toddler acting out the minute I sat down to nurse. I've since learned there were several things I ought to have done differently. Feeding time is the hardest for an older sibling because you need to give 100% attention to the new baby and it's hard to just jump up to take care of DS1's needs.
1. Anticipate when it'll be time to nurse (I nursed on demand and was still able to have a feel for "when") and take care of the older boy first. Get him a snack, a drink, then set him down nearby with a special basket/bin that has a puzzle or two, some NEW books (helpful if about siblings/babies, being helpful, etc.), and an interactive toy. This bin/basket is ONLY to be used during feeding times (whether nursing or not) and gets put away as soon as baby's done eating & burping. It keeps it special. Also, change or rotate the contents every week to keep it exciting
2. Keep in mind they turn two as soon as they hit 18 months! This will make sense in time, but truly, do extra toddler child proofing, because if DS1 starts to explore his limits while you're feeding, as mine did, you'll have to drop the baby into a swing a lot less often in order to "rescue" DS1 (or the house plant he's tipping over, etc.)
3. If you have a DVR and allow your DS to watch TV, record some shorts (e.g. Pingu, Curious George) from PBS/PBS Sprout. Save them to watch while you're feeding the baby and talk about them together.
4. The second baby is napping, spend a solid 15 minutes giving 100% undivided attention to DS1. Whatever chores or cleanup are necessary can wait! It's the best way to convey to the boys that they are #1 in your life. It's so easy to get sidetracked--"just a minute, honey, let me finish cleaning the kitchen"--and never get back to the activity your child's interested in.
Everyone's suggestions below are terrific--the sibling birth gift, the books to prepare for siblings, etc. We did both and it did help, but at that age, they'll do like to begin challenging themselves and their authority figures.
Good luck!